MELBOURNE, Australia -- Olympic champion Qinwen Zheng won't be repeating her surprising run to the Australian Open final in 2025 after a second-round upset loss to No. 97-ranked Laura Siegemund on Wednesday.

Zheng lost the 2024 championship match at Melbourne Park to Aryna Sabalenka and went on to win the Olympic gold medal in Paris and finish runner-up at the WTA Finals in a breakout season.

But her first tournament of the year ended in a 7-6 (3), 6-3 loss on John Cain Arena against 36-year-old Siegemund, who played aggressively from the first point and put Zheng off her game.

At 36, Siegemund is the third-oldest player to beat a WTA top-five player at the Australian Open in the past 30 years. The only one older: Serena Williams, who did so at 39 years old in 2021 (defeated No. 2 Simona Halep) and as a 37-year-old in 2019 (defeated No. 1 Halep).

Sabalenka, the two-time defending champion, extended her run to 16 wins at Melbourne Park by winning the last five games to beat No. 54-ranked Jessica Bouzas Maneiro 6-3, 7-5.

The score line didn't indicate the difficulty of the match, with Bouzas Maneiro taking huge swipes at the ball in her Australian Open debut and dictating some of the points against the world No. 1-ranked player. Her serve let her down, with Sabalenka able to relieve some pressure on her own serve with five breaks.

No. 7 Jessica Pegula had a 6-4, 6-2 win over Elise Mertens to reach the third round, along with Belinda Bencic and No. 23 Magdalena Frech.

Siegemund has never been past the third round in Australia, but is taking confidence from her big upset. Her only lapse was when she was broken serving for the first set. She recovered to dominate the tiebreaker, while Zheng remained too conservative in her tactics until right near the end.

"I knew I just had to play more than my best test. I had nothing to lose. I just told myself to swing free," Siegemund said. Zheng is "an amazing player. One of the best players right now, but I know I can play well and I wanted to show that to myself."

Siegemund became the first unseeded German woman to defeat a top-five seed at the Australian Open since 2003, when Marlene Weingartner ousted Jennifer Capriati in the opening round.

ESPN Research and The Associated Press contributed to this report.