MELBOURNE, Australia — Stan Wawrinka left it as late as possible before deciding his knee might just be good enough to get him through the Australian Open.

The 2014 champion was being way too ambitious. A 6-2, 6-1, 6-4 loss to No. 97-ranked Tennys Sandgren in the second round was his earliest exit in a decade at Melbourne Park.

“I only had surgery five months ago,” said Wawrinka, who still has a visible scar running down his left knee. “To be that far already, it’s more than what we could have expected.”

On a searing hot day at Melbourne Park when the temperature hit 104 Fahrenheit, No. 9 Wawrinka, No. 7 David Goffin and No. 13 Sam Querrey were the three seeded players to fall in the men’s draw.

Wimbledon champion Garbine Muguruza was among the five seeded women to lose on Day 4. And crowds were down on the previous two days. The forecast is for hotter weather on Friday.

Six-time champion Novak Djokovic survived the round, but complained of the “brutal” heat in a comeback four-set win over an ailing Gael Monfils that extended his record to 15-0 in their career head-to-heads.

The temperature had dropped to 29 C (84 F) by the time defending champion Roger Federer wrapped up his 6-4, 6-4, 7-6 (4) win over Jan-Lennard Struff just after 11:30 p.m. on Rod Laver Arena.

For a while Swiss compatriots Federer and Wawrinka were playing on adjoining courts at the same time, but their fitness and results were contrasting.

Wawrinka was appeared lethargic and generally didn’t threaten 26-year-old Sandgren, who had never beaten a top 10 player. His wins in the first two rounds here make Sandgren 2-0 at majors.

Wawrinka won here in 2014, at the French Open the following year and the U.S. Open in ’16. Until this week, he hadn’t played since Wimbledon last July.

“When you won three Grand Slams, you don’t feel great on the court like today,” he said. “But I need to take what I can to be positive with everything, with the big picture.”

Muguruza also struggled with her fitness in the build up to the Australian Open, hampered at two warmup tournaments by cramping and a thigh problem.

Blisters from the hot court surface didn’t help her in a 7-6 (1), 6-4 loss to No. 88-ranked Hsieh Su-wei, who has been No. 1 in doubles but had a career-high ranking of 23rd in singles.

That left only Maria Sharapova, Aneglique Kerber and French Open champion Jelena Ostapenko as Grand Slam winners in the women’s draw.

Sharapova and Kerber will meet in the third round, meaning only one of the remaining Australian Open winners can reach the second week.

Sharapova beat No. 14-seeded Anastasija Sevastova 6-1, 7-6 (4) to avenge a fourth-round loss in last year’s U.S. Open, the five-time major winner’s first Grand Slam after returning from a 15-month doping ban.

Kerber, the 2016 champion, had a 6-4, 6-1 win over Donna Vekic before the Margaret Court Arena crowd sang “Happy Birthday” to celebrate her 30th birthday.

“Happy to be playing tennis again like 2016,” said Kerber, who has won 11 straight matches.

Still aiming for a first major, top-ranked Simona Halep, a two-time French Open finalist, held off 2014 Wimbledon finalist Eugenie Bouchard 6-2, 6-2. Halep next plays Lauren Davis.

Ninth-seeded Johanna Konta lost in straight sets to U.S. lucky loser Bernarda Pera, who is ranked No. 123-ranked and making her Grand Slam debut. She didn’t even know she had a spot in the main draw until another player withdrew on Monday. Pera next plays No. 20 Barbora Strycova.

U.S. Open finalist Madison Keys opened with four straight aces and won the first 11 games in a 6-0, 6-1 win over No. 92-ranked Ekaterina Alexandrova.

Also advancing were No. 6 Karolina Pliskova, No. 8 Caroline Garcia, No. 26 Agnieszka Radwanska, No. 28 Ash Barty and No. 29 Lucie Safarova.

The heat peaked during Djokovic’s 4-6, 6-3, 6-1, 6-3 win over Monfils. Both players slouched over in the shade between points, earning warnings about time delays from the chair umpire.

The win was a relief for Djokovic, who lost in a second-round shocker here last year, but he said officials need to be more considerate about the players in the conditions.

Now seeded 14th, Djokovic needed four match points in an eight-minute last game before clinching the win against Mofils, who needed a medical timeout for heat stress and said he had trouble breathing.

“It was obvious we both suffered on the court today,” Djokovic said. “Really tough conditions — brutal.”

Djokovic, who won five of the six titles from 2011-2016, is a step closer to a possible meeting with fourth-seeded Alexander Zverev, who had a 6-1, 6-3, 4-6, 6-3 win over Gojowczyk.

Among the leading men advancing were 2009 U.S. Open champion Juan Martin del Potro, No. 19 Tomas Berdych, No. 21 Albert Ramos and No. 5 Dominic Thiem, who rallied to beat 190th-ranked American qualifier Denis Kudla 6-7 (6), 3-6, 6-3, 6-2, 6-3.

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