Laura Siegemund of Germany plays a backhand return to Zheng Qinwen of China during their second-round match at the Australian Open tennis championship in Melbourne, Australia. (AP )

MELBOURNE, Jan 15, (Xinhua): Chinese tennis star Zheng Qinwen crashed out of the Australian Open on Wednesday, but women's defending champion Aryna Sabalenka and men's third seed Carlos Alcaraz enjoyed comfortable second-round victories.

Ten-time tournament winner Novak Djokovic, second seed Alexander Zverev, and former US Open champion Coco Gauff were among the winners on a dramatic day four that started with Zheng's stunning exit.

Fifth seed Zheng's trusted serve let her down and she had no answers for German veteran Laura Siegemund, who prevailed 7-6 (3), 6-3 in two hours and 16 minutes on John Cain Arena.

It was a bitter disappointment for last year's finalist Zheng, whose hopes of a maiden Grand Slam triumph were undone by a first serve percentage of just 56 percent.

Zheng's exit has opened up the draw for Sabalenka, who progressed 6-3, 7-5 over Jessica Bouzas Maneiro. It was her 16th straight victory in Melbourne as Sabalenka's bid for a third straight Australian Open remained on track.

But Sabalenka has plenty of room left to improve after her normally reliable serve was broken three times by her 54th-ranked opponent.

"She played incredible tennis today, and it was a tough one," Sabalenka said of the 22-year-old Spaniard. "It's a good challenge to have and I love it. I have to play better, so this is the way I improve my game." Third seed Gauff continued her strong form after beating Jodie Burrage 6-3, 7-5 in one hour and 29 minutes. Gauff did endure struggles on serve in the second set and trailed 3-5 before winning the last four games.

Gauff will next play 30th seed Leylah Fernandez as she looks to continue her unbeaten start to this season after leading the US to the title at the United Cup.

Four-time Grand Slam champion Naomi Osaka came from behind to beat 20th seed Karolina Muchova 1-6, 6-1, 6-3 as she reached the third round of a major for the first time since 2022.

In the men's draw, a frustrated Djokovic overcame a second-set stumble to keep alive his bid for a record 25th Grand Slam title after beating Jaime Faria 6-1, 6-7 (4), 6-3, 6-2.

Djokovic was occasionally left flustered by the hard-hitting Faria, but his defensive mastery pulled him out of trouble.

His mood fluctuated throughout the three-hour contest, souring late in the second set when he complained about the lights from high above that were turned on amid increasingly gloomy skies.

But Djokovic regrouped and dominated after the roof of Rod Laver Arena was closed early in the third set. He will next face 26th seed Tomas Machac, who came from behind to beat Reilly Opelka in five sets.

While Djokovic has had to endure consecutive four-set matches, Alcaraz's red-hot start continued after blasting past Japan's Yoshihito Nishioka 6-0, 6-1, 6-4 in 81 minutes.

Zverev romped to a 6-1, 6-4, 6-1 victory over Pedro Martinez in the night session.

But sixth seed Casper Ruud became the biggest casualty in the men's draw after losing to 19-year-old Jakub Mensik, who fired 62 winners in the 6-2, 3-6, 6-1, 6-4 upset.

Men's top seed Jannik Sinner highlights day five action on Thursday, while Chinese women's players Wang Xiyu, Zhang Shuai and Wang Yafan will be playing their second-round matches.