Last year's Australian Open finalists Aryna Sabalenka and Zheng Qinwen both faced dramatic challenges in their second-round matches at the Australian Open on Wednesday. Sabalenka held on for the win, but Zheng became the first Top 10 seed to fall this week.

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World No.1 and two-time defending champion Sabalenka fended off 54th-ranked Jessica Bouzas Maneiro 6-3, 7-5 on Rod Laver Arena. Sabalenka had to fight back from 5-2 down in the second set before notching the 1-hour and 34-minute victory.

But German veteran Laura Siegemund caused the first major upset at the tournament, stunning No.5 seed Zheng 7-6(3), 6-3 on John Cain Arena, and denying a potential Sabalenka vs. Zheng rematch in this year's quarterfinals.

Siegemund shocker: Last year's Olympic gold medalist Zheng came into the event as a possible title contender at a career-high ranking of No.5, but the guile of World No.97 Siegemund won out as she clinched her 11th career Top 10 win after 2 hours and 16 minutes of play.

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Former Top 30 player Siegemund came into this match having posted four Top 5 wins in her career -- although all of those previous Top 5 upsets had come at Stuttgart, her career-best tournament where she was champion in 2017 and runner-up in 2016.

On Wednesday in Melbourne, though, Siegemund was performing as if she was on her beloved indoor clay of Stuttgart's Porsche Arena. Firing return winners almost at will and finding passing winners with ease, Siegemund served for the first set at 5-4.

Zheng was able to battle back and push the first set into a tiebreak, but the German would not be denied. Siegemund successfully read most of the power plays from Zheng throughout, and she won the last five points of the breaker to light up the upset alert.

Siegemund had no trouble moving through the second set, ousting the Chinese No.1 and reaching the Australian Open third round for the second time in her career. Playing some of her most aggressive tennis, the German matched big-hitting Zheng in winners, with 29 apiece.

"Obviously I'm very happy and very proud of my performance," Siegemund said afterwards. "I expected a very, very tough game against this kind of top player. I think she's one of the best in the world.

"It's never going to be easy, and there will be continuously through the match, like, tough situations to solve. But I wanted to come out, you know, courageous. I wanted to show the variety of my game, be very aggressive, and that's what I did."

With her massive upset, Siegemund is the first German woman to claim a Top 5 win at the Australian Open since Angelique Kerber defeated Serena Williams in the 2016 final.

Sabalenka survives: Meanwhile, top-seeded Sabalenka needed to battle before collecting her 16th straight win at Melbourne Park. After a routine first set, Sabalenka found herself down 5-2 in the second.

Bouzas Maneiro took the big second-set lead while she was aiming for her second upset of a Grand Slam defending champion. The 22-year-old Spaniard toppled defending champion Marketa Vondrousova on Centre Court at Wimbledon last summer.

Sabalenka, though, was able to wrest back the momentum, winning five games in a row to swipe the second set and book her spot in the third round for the fifth straight year. She is the first woman to win 16 straight Australian Open matches since Victoria Azarenka in 2014.

"She played incredible tennis, especially in the second set," Sabalenka said. "I'm really happy I was able to turn around that set.

"There's so many players who are playing really well in these conditions ... Today's match proved that. Girls can go there and just play without any fear, without anything to lose. They can put you in really uncomfortable position. You have to go out there and fight and compete and show your best."

With the win, Sabalenka remains in the mix to retain her World No.1 ranking coming out of Melbourne, although Iga Swiatek and Coco Gauff also have the possibility to claim the top spot after the Australian Open.

Sabalenka is also attempting to become the first woman to win three straight Australian Open singles titles since Martina Hingis from 1997 through 1999.

Pegula, Andreeva advance: Two other top players, No.7 seed Jessica Pegula and No.14 seed Mirra Andreeva, also clinched spots in the third round on Wednesday afternoon -- but they took very different paths to get there.

Pegula breezed past former Australian Open semifinalist Elise Mertens 6-4, 6-2. Pegula, who reached three straight Australian Open quarterfinals between 2021 and 2023, needed 1 hour and 11 minutes to best Mertens, the highest-ranked unseeded player.

Although that result went to form on paper, it was a big win for Pegula. The American had never beaten Belgium's Mertens in their three previous meetings.

By contrast, 17-year-old Andreeva needed to go the absolute distance before overcoming 63rd-ranked Moyuka Uchijima of Japan 6-4, 3-6, 7-6(8). Uchijima led 5-3 in the deciding set before the teenager stormed back to eke out a 2-hour and 19-minute win.