In pics: Highlights of Day 4 of the Australian Open

Wawrinka stumbles, Serena cruises, and other news from Day 4 of the year's first Grand Slam. 

Canada's Milos Raonic dropped the first set but recovered to beat Stan Wawrinka 6-7(4), 7-6(6), 7-6(11), 7-6(5) in a contest that lasted just over four hours.

Serena Williams, chasing a record 24th Grand Slam title, struck 20 winners and converted six break points to beat Canadian Eugenie Bouchard 6-2, 6-2.

The American, who recorded her 83rd match win at Melbourne Park, will face Ukraine's Dayana Yastremska in the third round.

World number one Simona Halep survived an almighty scare against Sofia Kenin before she overcame the unseeded American 6-3, 6-7(5), 6-4 to reach the third round. 

After blasting her way through the first set and racing to a 3-0 lead in the next, 2018 runner-up Halep appeared to step off the gas as she allowed the determined Kenin to level at 3-3 and eventually drag the contest into a decider. 

Halep battled hard in the third set and regained control with a decisive break in the ninth game before closing out the match on her second match point in two hours and 31 minutes.

Seven-time Grand Slam champion Venus Williams made a late charge into the Australian Open third round, grinding out a win in her second three-set victory of the tournament. 

The unseeded 38-year-old American came back strongly to beat Alize Cornet of France 6-3, 4-6, 6-0, setting up a meeting with top seed Simona Halep.

The US Open champion Naomi Osaka powered past unseeded Slovenian Tamara Zidansek 6-2, 6-4.

She plays Taiwan's Hsieh Su-wei in the next round. 

Kei Nishikori was dragged into a five-setter for the second match in a row but the eighth seed dug deep to beat Croatian veteran Ivo Karlovic 6-3, 7-6(6), 5-7, 5-7, 7-6(7). 

The Japanese looked in control after taking a two-set lead but the 39-year-old Karlovic did not back down and levelled the contest.

Nishikori held his nerve in a tight decider to prevail in three hours and 48 minutes.

Australian wildcard Alexei Popyrin advanced to the third round after seventh seed Dominic Thiem retired from the contest while trailing 7-5, 6-4, 2-0.