MELBOURNE –

World No. 1 Iga Swiatek battled back from two breaks down in the final set to beat Danielle Collins 6-4, 3-6, 6-4 to reach the third round of the Australian Open for the fifth consecutive year on Thursday.

Alexander Zverev, the No. 6 seed on the men's side, also survived a scare, digging deep to beat Slovakian qualifier Lukas Klein 7-5, 3-6, 4-6, 7-6 (7-5), 7-6 (9-7) to advance.

Collins is ranked 62nd in the world, but she has a good record at Melbourne Park and beat Swiatek in the semifinals two years ago before losing to local favorite Ash Barty in the final.

Swiatek, a four-time Grand Slam winner, has an excellent record against lower-ranked players but had to dig deep into her reserves to overcome Collins in a dramatic, scrappy three-hour contest at Rod Laver Arena.

The 30-year-old Collins' serve faltered at key moments in the match, and Swiatek found her best tennis late in the decisive set to book a third-round clash with Czech Linda Noskova.

"Oh my god, honestly, I was already at the airport," said Swiatek, whose run to the 2022 semifinals is her best showing at Melbourne Park.

"I wanted to fight until the end, she played perfectly, but it would be hard for anybody to keep that level, so I wanted to be ready when mistakes were going to come from the other side.

"I wanted to push then and I did that at the end and I'm really proud of myself because it wasn't easy."

Collins drew first blood with a break for 3-1 in the opening set, but Swiatek broke straight back to love and the match was locked up at 3-3 when a shower interrupted play for 30 minutes as the roof was closed.

Swiatek came out firing after that, racing around the court to win four straight games to secure the first set and go a break up in the second.

That seemed to galvanize Collins, who ramped up the speed of her groundstrokes to rattle off the next five games and serve for the second set.

"I felt I had the momentum going and she suddenly started playing two times faster," Swiatek said.

"I had no idea how to react to that for a couple of games but I just came back and thought the only thing I can focus on is myself."

Five double faults helped Swiatek save four set-points and the Pole fought off another on her own serve before Collins finally got over the line on her sixth attempt to even the contest after more than two hours on court.

Collins was not done yet, and two breaks gave her a 4-1 lead in the decider, but that was as good as it got for the American, who raced straight off the court at the end and later announced she would be retiring this year.

"I lost 6-4 in the third to one of the best players in the world, and she played some great tennis," Collins said. "Left it all on the court.

"I don't really know exactly when, but this will be my last season and I'm really looking forward to that."

The rain also forced Zverev and Klein to finish their match under the roof at John Cain Arena, and Zverev found himself in big trouble as Klein, the world No. 163, turned up the pressure after losing the opening set by winning the next two.

"I would have much rather won in an hour-and-a-half but what can I do," Zverev said. "He played incredible, was hitting every ball as hard as he could from both sides. I didn't really know what to do a lot of the time.

"To be honest, he probably deserved to win more than me. That's how tennis goes sometimes. I'm happy that I'm through but his ranking is no value to how he's playing."

The 25-year-old Klein, whose only previous appearance in the main draw at a Grand Slam was at Wimbledon in 2022, was inching toward his first career top-10 win but squandered the chance to break at 4-4 in the fourth set.

"I was thinking there's a Qantas flight at 11 p.m. tonight straight to Dubai and then one to home," Zverev said, adding that he was just a spectator much of the time as Klein reeled off the winners, finishing with 80 in total.

"I was playing but the match was his hands, he was playing winners or he was missing ... I'm extremely happy to be honest."