Melbourne, Australia – Serena Williams survived a challenge from Simona Halep in the fourth round of the Australian Open, defeating the top-ranked player in the world 6-1, 4-6, 6-4.

Williams was pushed to a third set for the first time in the tournament, after a surge from Halep, who lost all of her service games in the first set, which lasted just 20 minutes.

"I felt like I'd been hit by a train," Halep said.

The French Open champion appeared to read Williams better in the second, improving the accuracy of her serves and becoming more aggressive, breaking the Williams at 5-4 to claim the second set.

She also had the 23-time Grand Slam champion on the ropes at 3-2 in that set, forcing Williams to save three breakpoints in a game that lasted 10 minutes before Williams held serve and leveled.

Williams broke again to pull ahead 5-3 in the third set, and held serve to claim the set and the match.

"I really needed to elevate my game, she's the No. 1 player in the world and there's a reason why," said Williams.

Other women's matches

The women's quarterfinals began late Monday with Anastasia Pavlyuchenkova playing American Danielle Collins and No. 8 Petra Kvitova taking on No. 15 Ashleigh Barty.

Collins, a two-time NCAA champion, was 0-5 in majors until this tournament. Barty is the first Australian woman in a decade to reach the quarterfinals at Melbourne. It's been 41 years since an Aussie won the women's title.

Williams will play No. 7 Karolina Pliskova, and No. 4 Naomi Osaka will meet No. 6 Elina Svitolina.

Osaka, the reigning U.S. Open champion, came from a set down in the fourth round to defeat Anastasija Sevastova 4-6, 6-3, 6-4.

Tsitsipas reaches semis

No. 14 Stefanos Tsitsipas became the first men's player to reach the semifinals when he defeated No. 22 Roberto Bautista Agut 7-5, 4-6, 6-4, 7-6 (2).

Tsitsipas, who stunned Roger Federer in his previous match, is the first player from Greece to reach the last four at a major.

At 20 years, 168 days, he is the youngest man to make the semifinals at a major since Novak Djokovic at the 2007 U.S. Open. He's the youngest man to do so in Australia since Andy Roddick in 2003.

"I'm just living the dream," Tsitsipas said.

The other quarterfinals lined up like this: No. 2 Rafael Nadal vs. American Frances Tiafoe, No. 16 Milos Raonic vs. No. 28 Lucas Pouille and No. 1 Novak Djokovic vs. No. 8 Kei Nishikori.

Nadal hasn't dropped a set yet and hasn't lost to an American at a major since 2005.

Raonic defeated fourth-seeded Alexander Zverev in the fourth round in spectacular fashion 6-1, 6-1, 7-6 (7-5). Zverev committed 10 double faults and fired just six aces

Djokovic advanced with a 6-4, 6-7 (5-7), 6-2, 6-3 victory in three hours over Daniil Medevdev. "Definitely a physical battle," Djokovic said.

Nishikori defeated Pablo Carreno Busta in a five-set marathon 6-7 (8-10), 4-6, 7-6 (7-4), 6-4, 7-6 (10-8).