MELBOURNE — Defending champion Roger Federer coolly swept into the Australian Open third round Thursday as Novak Djokovic survived a grueling fitness test under the brutal Melbourne sun.

But with temperatures touching an energy-sapping 40 Celsius (104 Fahrenheit) and ice-towels in use, third seed Garbine Muguruza and 2014 winner Stan Wawrinka both wilted and were knocked out.

Maria Sharapova, in contrast, laid down her title credentials with dominant win against 14th seed Anastasija Sevastova. World No. 1 Simona Halep was also convincing in her straight demolition of Canadian Eugenie Bouchard. Halep allayed fears over the ankle injury she suffered in the opening round with a convincing 6-2, 6-2, victory over Bouchard. "I felt the pain but I didn't think about it," said Halep, who will next face American Lauren Davies.

Swiss 19-time Grand Slam champion Federer disposed of Jan-Lennard Struff 6-4, 6-4, 7-6 (7-4), avoiding the worse of the furnace-like conditions in his night match. "I practice with him so I had the information I needed," he said, adding that the heat doesn't bother him. If you want to get to the top, you've got to play in all conditions."

It was never going to be easy for the German. While Federer had made at least the third round each year since his Melbourne Park debut in 2000, Struff had failed to go further than the second round at any Grand Slam.

Six-time Melbourne champion Djokovic had a much tougher time, coming through a four-setter against Gael Monfils in a thorough examination after being out for six months with an elbow injury. He lost the first set before Monfils began struggling in the second and was heard at one point telling the umpire: "I'm tired and dizzy."

The Frenchman remarkably recovered to make a fight of it before a relieved Djokovic, seeded 14, came through 4-6, 6-3, 6-1, 6-3. "It was tough conditions, brutal, especially the first 90 minutes," he said. "I'm still not 100 percent, but building. I have a lot of faith and self-belief."

The heat took its toll on a host of players, including Wimbledon champion Muguruza, who suffered with heat-blistered feet. She was knocked out by 88-ranked Hsieh Su-wei from Taiwan 7-6 (7-1), 6-4 but refused to make excuses. "I maybe could have done things better, but at the end, she deserves to win," she said.

Wawrinka also failed to progress, clearly still struggling from a lengthy knee injury lay-off in his straight sets defeat by American Tennys Sandgren, who eased to a 6-2, 6-1, 6-4, win at Melbourne Park. Sandgren, 26, has made it to the third round of a grand slam for the first time and will take on German Maximilian Marterer, who ousted Fernando Verdasco 6-4, 4-6, 7-6(7-5), 3-6, 6-3.

Russian drawcard Sharapova was on Rod Laver Arena early and avoided the brunt of the weather as she showed why she should be again taken seriously in dismantling Sevastova 6-1, 7-6 (7-4).

It was the Latvian who ended her Grand Slam comeback in the last 16 of the US Open in 2017 as she returned from a drug ban. But not this time. Sharapova, a five-time Grand Slam winner and the 2008 Australian champion, was unstoppable in racing through the first set before a battle in the second.

"You know, it was a warm day. I did my job in two sets against someone that's been troubling in the past for me," she said afterwards. "I think I deserve to smile out there after that victory."

Being unseeded means she has her work cut out to reach another final. Next up is in-form 2016 Australian Open champion Angelique Kerber, who beat Donna Vekic in two easy sets as she starts to rediscover her best again after a miserable last year.

Eighth seed Caroline Garcia also stayed in the title hunt, but she found it hard going against Czech teen Marketa Vondrousova, who pushed her to three grueling sets. "My feet are burning," she said. "But we know it's like this in Australia — the next day it can be freezing."

Dangerous sixth seed Karolina Pliskova also went through, dropping just two games against Beatriz Haddad Maia. Two-time Australian Open semifinalist Agnieszka Radwanska rallied from a set and a break down to beat Ukraine's Lesia Tsurenko 2-6, 7-5, 6-3 to reach the third round.

Ashleigh Barty, backed by a roaring home support on Rod Laver Arena, rallied from a set down to beat Italian Camila Giorgi 5-7, 6-4, 6-1 in the second round. She will next face Japan's Naomi Osaka, who defeated 16th seed Elena Vesnina 7-6(7-4), 6-2. American Madison Keys, a US Open finalist last year, took just 41 minutes to beat Russian Ekaterina Alexandrova 6-0, 6-1, in the second round. Briton Johanna Konta, ranked 10th in the world, crashed out to world No. 123 Bernarda Pera in the second round, with the American winning 6-4, 7-5 defeat.

Among men into the third round was fifth seed Dominic Thiem, who came from two sets down to beat American qualifier Denis Kudla, 6-7(8-6), 3-6, 6-3, 6-2, 6-3 in the second round, and fourth seed Alexander Zverev. Young gun Zverev powered into the third round with a solid win over fellow German Peter Gojowczyk. The world No.4, just 20, took almost two hours to put away the 62nd-ranked Gojowczyk 6-1, 6-3, 4-6, 6-3 on Hisense Arena.

Zverev will face another "NextGen" star Chung Hy-eon of South Korea in the next round on Saturday. While Zverev won five titles and was one of just four players to beat Roger Federer last year, he has yet to get beyond the fourth round at a Grand Slam.

He was ousted in the third round in Melbourne in 2017 and will be seeking to go further this campaign. It was an impressive performance by Zverev with 25 aces and no double faults, 46 winners and five service breaks.

Two-time semifinalist Tomas Berdych recorded 42 winners, including 12 aces, to overcome Spaniard Guillermo Garcia-Lopez 6-3, 2-6, 6-2, 6-3, in the second round.

Former US Open champion Juan Martin Del Potro fought off rising Russian Karen Khachanov as he battled through the pain barrier to reach the third round. The Argentine 12th seed needed 3hr 45 min in oppressive court conditions to see off the 47th-ranked youngster 6-4, 7-6 (7-4), 6-7 (0-7), 6-4.

Del Potro, 29, in his first appearance in Melbourne for four years after wrist injuries, will face Czech veteran Berdych in the third round.

Del Potro, who reached the third round in Melbourne for the fifth time, is looking to improve in his quarterfinal appearances in 2009 and 2012, losing to Roger Federer both times.

The Argentine, who upset Federer in the 2009 US Open final, came into Melbourne a losing finalist to Roberto Bautista Agut in Auckland this month. French veteran Julien Benneteau recovered from a flat start to beat seventh seed David Goffin 1-6, 7-6(7-5), 6-1, 7-6 (7-4) in the second round. — Agencies