Wimbledon champion Garbine Muguruza struggled with the heat in an upset second-round loss to Hsieh Su-wei that left just three major winners in the Australian Open women's draw on Day 4.
Two of them will meet in the third round.
With temperatures forecast to hit 39 Celsius (102 Fahrenheit), 2008 Australian Open winner Maria Sharapova hurried through the first set on Rod Laver Arena in 23 minutes before beating No. 14-seeded Anastasija Sevastova 6-1, 7-6 (4).
Heat hovered around 40 C (104 F) as 2016 champion Angelique Kerber set up a meeting with Sharapova with a 6-4, 6-1 win over Donna Vekic in a 70-minute afternoon match at Margaret Court Arena.
The crowd sang "Happy Birthday" and gave three hoorays to celebrate Kerber's 30th birthday. It was her 11th win, including the Sydney International title last week.
"Happy to be playing tennis again like 2016," said Kerber, who planned an ice bath before a relaxed dinner out in Melbourne to mark the occasion.
Six-time champion Novak Djokovic extended his personal streak against Gael Monfils to 15 consecutive wins with a 4-6, 6-3, 6-1, 6-3 victory in which both players slouched and crouched over in the shade at the back of the court between points.
The win was a relief for Djokovic, who says he's still not 100 percent after six months off the tour with an injured right elbow and who lost in a second-round shocker here last year.
Now seeded 14th, Djokovic needed four match points in an eight-minute last game before clinching the win.
"It was obvious we both suffered on the court today," Djokovic said. "Really tough conditions - brutal."
It's supposed to be even hotter on Friday, with a forecast high of 42 Celsius (108 Fahrenheit).
Sharapova, who missed last year's Australian Open while serving a 15-month ban for a failed doping test on her previous trip to Melbourne Park, was happy to beat the player who'd ended her run at the U.S. Open last year - her first major since the suspension.
"I did my job in two sets against someone that's been troubling in the past for me," Sharapova said. "I think I deserve to smile out there after that victory."
Muguruza had little to celebrate in a 7-6 (1), 6-4 loss to No. 88-ranked Hsieh, who has been No. 1 in doubles but had only previously beaten one top 10 player in singles.
Muguruza followed Venus Williams and U.S. Open champion Sloane Stephens out of the tournament in the first week, leaving only Sharapova, Kerber and French Open champion Jelena Ostapenko as Grand Slam winners in the draw.
Muguruza acknowledged that the heat was bothersome but it wasn't the hottest conditions she'd experienced in Australia.
"I could have done things better," she said, "but at the end, she deserves to win."
Considered one of the contenders for the title after Serena Williams opted against defending her Australian title, Muguruza had a troubled preparation including a retirement at the Brisbane International and a withdrawal ahead the quarterfinals in Sydney because of a right thigh injury.
Against Hsieh, she struggled with her serve, made 43 unforced errors. She needed a medical time out in the first set, and accidently hit a ball into a line judge in frustration.
Ninth-seeded Johanna Konta, the 2016 Australian Open semifinalist, was upset in straight sets by U.S. lucky loser Bernarda Pera.
No. 123-ranked Pera is making her Grand Slam debut and, after losing in the last round of qualifying, didn't even know she had a spot in the main draw until Margarita Gasparyan withdrew.
"It feels amazing. I was ready to leave on Monday and then they told me I'm in, so I was obviously excited," Pera said. "I was checking the tickets to fly back. I'm happy I didn't buy one."
Pera will next play No. 20 Barbora Strycova.
U.S. Open finalist Madison Keys was done and dusted in 41 minutes, opening with four straight aces and winning the first 11 games in her 6-0, 6-1 win over No. 92-ranked Ekaterina Alexandrova.
Also advancing were No. 8 Caroline Garcia and No. 26 Agnieszka Radwanska, who next plays Hsieh.
Lauren Davis beat Andrea Petkovic 4-6, 6-0, 6-0 and has a potential third-round match against top-ranked Simona Halep, who had a later match against Eugenie Bouchard.
Former No. 1-ranked Karolina Pliskova advanced to a third-round match against No. 29 Lucie Safarova.
No. 13 Sam Querrey was the latest of the U.S. men eliminated when he lost his second-round match 6-4, 7-6 (6), 4-6, 6-2 to 80th-ranked Marton Fucsovics.
No. 5 Dominic Thiem rallied to beat 190th-ranked American qualifier Denis Kudla 6-7 (6), 3-6, 6-3, 6-2, 6-3 and No. 21 Albert Ramos beat another American, wild-card entry Tim Smyczek.