From Cristiano Ronaldo's struggle to Lionel Messi's double, major takeaways from UEFA Champions League quarter-finals second-leg

Tottenham will play Ajax and Barcelona will clash with Liverpool in the semi-final of UEFA Champions League, here are the things learned from a sensational quarter-final second leg

Lionel Messi extended his incredible record of tormenting Premier League clubs in the Champions League.

Messi has now scored 24 goals in 32 Champions League games against English opponents, more than any other player in the tournament.

Accelerating away from Fred, Messi was granted too much space by United defender Chris Smalling on the edge of the area and took full advantage with a superb low shot that flashed past David de Gea in the 16th minute.

It was a sweet moment of revenge for Messi, who suffered a facial injury in the first leg after a clash with Smalling.

Messi's first Champions League quarter-final goal since April 2013 ended a run of 12 games at this stage without netting.

Just four minutes later, Messi effectively clinched Barca's return to the semi-finals for the first time since 2015 with a helping hand from United keeper De Gea, whose woeful attempted save failed to stop the Argentine's weak shot slip under his body.

Messi has 45 goals in 41 games in all competitions this season, with his 10 in the Champions League making him the tournament's leading scorer in 2018-19.

With Messi on their side, it's no wonder Barca are now unbeaten in 31 Champions League home games.

Liverpool remain the only Premier League club to win at the Camp Nou in 24 attempts dating back to February 2007.

For the first time since 2010, Cristiano Ronaldo will not be in the Champions League semi-finals.

And he held his head in his hands in despair as he slumped to the pitch at the Allianz Stadium.

He had single-handedly lifted Juventus into the quarter-final with a stunning hat-trick in the second leg against Atletico Madrid.

But he was unable to lift his team to a Champions League title he had won in the three previous seasons and in 2014 with Real Madrid, as well as with Manchester United 11 years ago.

He holds the record for Champions League goals with 126. That total includes 65 goals in the knockout rounds, another record.

Son Heung-min has proved this season that playing without Harry Kane won't be fatal to Tottenham's ambitions and the South Korea forward was in sparkling form with a brilliant double.

With Tottenham trailing after just four minutes and Kane sidelined, Son set about dismantling City's defence.

Seizing on Aymeric Laporte's miscued block, Son quickly whipped a low strike that caught Ederson off balance as it bounced in off the City goalkeeper's foot for the seventh minute equaliser.

That was followed by Son's second three minutes later as he shifted away from Kyle Walker before curling a superb shot into the far corner.

Son, who had scored the first leg winner against City, has now bagged 11 goals in 11 matches with Kane absent this term.

His pace, skill and astute movement make him a different proposition for central defenders than Kane and some have even suggested Tottenham look better with Son up front than the England captain.

However, there was a sting in the tail as a second half booking for Son, who tugged on Kevin De Bruyne on the edge of the Tottenham area, means he will miss the first leg of the semi-final.

Philippe Coutinho has endured a difficult time since moving to Barcelona for £142 million ($185 million) in January 2018, but the former Liverpool forward gave a timely reminder of his quality with a stunning strike to wrap up the victory.

Substituted during Barca's recent win over Rayo Vallecano at the Camp Nou, Coutinho's disappointing form reached a new low when he was whistled by a small section of his own fans.

But the Brazilian was handed a start against United as one of Barca's front three instead of Ousmane Dembele, who has just recovered from a hamstring injury, and at last he showed a glimpse of the talent that made him such a favourite of United's rivals at Anfield.

With Barca already well on the way to the last four by the 61st minute, Coutinho had nothing to lose when he unleased a majestic curling effort that raced into the top corner from 20 yards.

Coutinho seized the opportunity to emphasise that he won't be affected by the doubters, celebrating the goal by sticking his fingers in both ears while staring at the crowd in a defiant gesture.

After a series of frustrating defeats when the stakes were at their highest, Mauricio Pochettino's Tottenham finally came through on the big occasion.

Falling short in FA Cup semi-finals, Champions League knockout games and Premier League title battles had tarnished Pochettino's otherwise impeccable credentials as one of the brightest young managers in Europe.

But, playing without injured star striker Harry Kane, Pochettino inspired a tour de force performance from Tottenham.

Pochettino is yet to win his first major trophy, but he out-witted City boss Pep Guardiola, who has won every prize the game can offer.

Tottenham were aided by two huge VAR decisions that allowed to them to score the decisive third goal when Fernando Llorente bundled in off a combination of his arm and hip -- and then escape what would have been a Raheem Sterling last-gasp winner as offside was given against Sergio Aguero.

But luck wasn't the reason for Pochettino's success as he leads Tottenham in the Champions League semi-finals for the first time despite not buying a single player in the last two transfer windows due to the restraints from the move to their new stadium.

Tottenham are in the last four of a European competition for the first time since the 1984 UEFA Cup because Pochettino and no wonder Tottenham's jubilant fans chanted Pochettino's name loudly as he danced with his players on the pitch at full-time.

Pep Guardiola held his head in his hands when Ilkay Gundogan's close-range effort ballooned over the bar in the closing moments, the Manchester City manager's angst at his latest Champions League failure clear for all to see.

Guardiola is rightly regarded as the best manager of his generation after trophy-laden spells with Barcelona, Bayern Munich and now City.

But despite having access to vast financial backing at all three clubs and coaching some of the world's greatest players, Guardiola has only got his hands on the Champions League trophy twice in his 11-year career.

The Spaniard won it with Barcelona in 2009 and 2011, while also earning a winners' medal as a Barca player in 1992.

Dumped out by Liverpool last year, City, who have never won the Champions League, once again came up short on the big occasion in Europe.

City's bid for an unprecedented quadruple is over and now the question is whether they can recover in time to overhaul Liverpool in the Premier League title race.

Spurs could ruin that City dream as well when they return to the Etihad Stadium in the league on Saturday.

Barcelona's Camp Nou will always hold a special place in Ole Gunnar Solskjaer's heart, but the Manchester United manager won't regard the famous old arena with quite as much fondness after this frustrating loss.

It was the first time Solskjaer had played or managed at the Camp Nou since scoring the treble-clinching winner for United in the final seconds of the 1999 Champions League final against Bayern Munich.

Twenty years after the goal he has seen a "million times" since, lightning didn't strike twice for the Norwegian.

"We are confident but humble enough to know it's our hardest night of the season," he said in a prescient comment just before kick-off.

United were aiming to overturn another first-leg deficit after their epic effort in recovering from 2-0 down to beat Paris Saint-Germain on away goals in the last 16.

But, crucially, Marcus Rashford hit the bar with a glorious chance to give United the lead inside 40 seconds and it wasn't long before Lionel Messi put the tie beyond Solskjaer's reach.

Ajax Amsterdam coach Erik ten Hag said his team would keep "pushing boundaries" after they brought a breath of fresh air to the Champions League by reaching the semi-finals on Tuesday.

Having knocked out Real Madrid with a 4-1 win at the Bernabeu in the previous round, Ajax came from behind to beat Juventus 2-1 in Turin and complete a remarkable 3-2 aggregate win in their quarter-final.

In doing so, they became the first team to reach the Champions League semi-finals after playing through three qualifying rounds.

Ajax began their campaign in the second qualifying round on July 25 -- 10 days after the World Cup final -- at home to Austrian side Sturm Graz, a game which attracted more than 50,000 fans.

Even Cristiano Ronaldo was not enough for Juventus to end their 23-year wait for the European title as they crashed out in the Champions League quarter-finals for a second consecutive year.

Juventus splashed out 100 million euros ($117 million) on the five-time Champions League winner last summer and the Portuguese star headed in his 126th Champions League goal to put the home team in front on Tuesday.

But a devastating young Ajax team, spearheaded by 19-year-old defender Matthijs De Ligt, ran riot in the second half in Turin to seal a famous 3-2 aggregate victory.

Fernando Llorente sent Tottenham into their first European Cup semi-final in 57 years and ended Manchester City's quadruple hopes by scoring the decider in a breathtaking Champions League game that produced seven goals and saw an apparent injury-time winner for City ruled out after a video review.

City fans had celebrated what looked like the goal that sent them into the last four before the replay confirmed that Sergio Aguero was offside in the build-up.

It was a perfect example of the benefits of using technology but it was also hard on Pep Guardiola's side who had been the stronger over the 90 minutes.

"It is cruel but it is what it is and we have to accept it," said Guardiola, whose dream of winning four titles this season is now over.

Liverpool face a testing Champions League semi-final against Barcelona, but have made a statement by reaching the last four for the second straight year and are equipped to deal with the very best, manager Juergen Klopp said.

Liverpool beat Porto 4-1 at the Dragao Stadium on Wednesday to advance 6-1 on aggregate with their attacking trio of Mohamed Salah, Sadio Mane and Roberto Firmino all on target once again.

With their defence marshalled by the hugely impressive Virgil Van Dijk, Liverpool, who reached the final last year, are now undefeated in their last 17 games in all competitions as they seek a Premier League-Champions League double.

"We have been together for a while, we needed to be more mature," Klopp said.

"The boys made their own experience in this competition, especially last year. It's the second time in a row we are in the semis.

"That says something, it's a big statement for the club. OK we play Barca, maybe it's not too easy," the German added with a smile.