– Roger Federer showed his full-on championship form Thursday, but Stan Wawrinka was felled by a post-surgery knee as the Swiss former Australian Open champions split the difference in the second round.

The pair got lucky from a weather standpoint, playing cooler (88-degree) night matches after 104-degree summer daytime temperatures.

Federer, in the hunt for a sixth Melbourne title and 20th Grand Slam singles title, won 6-4, 6-4, 7-6 (7-4) against German Jan-Lennard Struff.

But 2014 Melbourne winner Wawrinka showed that he still needs more recovery time after his summer knee operation, falling to American Tennys Sandgren 6-2, 6-1, 6-4 in 88 minutes with 35 unforced errors.

"I was struggling with everything, not the knee especially, the knee was still handling well," said Wawrinka, a three-time Grand Slam winner.

"As long as I enter on the court, and I don't feel any big injury, I'm not going to retire [from a match]. I was here to fight and to try everything I could."

Six-time champion Novak Djokovic showed iron-man stamina as he battled hothouse conditions to overcome fatigued Frenchman Gael Monfils 4-6, 6-3, 6-1, 6-3.

On the women's side, top seed Simona Halep successfully tested an injured ankle under match conditions, producing a 6-2, 6-2 defeat of Canadian Eugenie Bouchard.

Halep rolled the ankle in her first-round win and was unsure of playing before making the call to go ahead.

"I still feel pain, and I couldn't practice much," she said. "But during the match, I just forgot about it. I had a very tight tape.

"The most important thing is that I could play my game not thinking about the ankle. I did that great today."

Taiwan's Hsieh Su-Wei claimed the biggest upset so far, knocking out third seed Garbine Muguruza 7-6 (7-1), 6-4.

Muguruza was one of the five seeded women's players who were eliminated on Day 4. That left only Maria Sharapova and Angelique Kerber — who will meet in the next round — and French Open champion Jelena Ostapenko as Grand Slam winners in the women's draw.

Sharapova beat 14th-seeded Anastasija Sevastova 6-1, 7-6 (4) to avenge a loss in last year's U.S. Open, the five-time major champion's first Grand Slam after returning from a 15-month doping ban.

Kerber, the 2016 Australian Open champion, had a 6-4, 6-1 win over Donna Vekic before the crowd sang "Happy Birthday" to celebrate her 30th birthday.