China's Wang Qiang return a backhand shot to Serena Williams of the US in their third round singles match at the Australian Open tennis championship in Melbourne, Australia, Jan 24, 2020. (LEE JIN-MAN / AP)

MELBOURNE - China's Wang Qiang toppled tennis legend Serena Williams at the Australian Open on Friday, while defending champion, Naomi Osaka bowed out to 15-year-old Coco Gauff in the third round.

I think my team always believed that I can do it. After last time I did really hard work on and off the court so I think it's really good work.

Wang Qiang, tennis player

In a day of upsets for the champions, Williams lost to Wang, 6-4, 6-7 (2), 7-5, and Osaka crumbled to youngster Gauff, 6-3, 6-4.

Meanwhile former world No 1 Caroline Wozniacki ended her tournament, and her career, after losing to Tunisian, Ons Jabeur, 7-5, 3-6, 7-5.

The Danish Grand Slam winner always planned to retire after this year's event and said that the reception from fans and her family after the match provided a fitting end.

"It was a packed stadium, people were clapping, I had the Danish flag at my back and I had my family there," Wozniacki said.

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"I don't think I could have scripted it any better. I think it was the perfect moment."

At the same time Wozniacki was ending her illustrious career, long time friend and competitor, Williams was suffering the tournament's biggest upset so far at the hands of China's No 1 player, Wang.

The last time the pair met was at the 2019 US Open quarterfinals, when Williams demolished the Chinese contender 6-1, 6-0.

Despite being ruled out by most observers, Wang showed early on that she meant business, moving Williams around the court and withstanding the champion's devastating returns.

Wang said that time in the gym along with belief in herself, and from her team, helped her play a vastly different match this time around.

Japan's Naomi Osaka reacts during her third round loss to Coco Gauff of the US at the Australian Open tennis championship in Melbourne, Australia, Jan 24, 2020. (LEE JIN-MAN / AP)

"I think my team always believed that I can do it. After last time I did really hard work on and off the court so I think it's really good work," Wang said in a post-match interview.

"During the second set I was a little bit confused because I lost, but I had to be calm...focus on every point and trust myself."

Meanwhile, 15-year-old sensation, Gauff defeated reigning champion Osaka with the youngster also overcoming a previous crushing loss to her opponent, going down 6-3, 6-0 in New York last year.

Despite her age, and last year's result, Gauff dispatched Osaka in only a little over an hour, displaying a more developed serve which took the Japanese third seed by surprise.

"You don't want to lose to a 15-year-old, you know, but I guess that's for me a reality check," Osaka said after the match.

"It doesn't really matter the age of the opponent, of course she deserves to be here, her serve is way better than I played her last year."

In the men's draw, Roger Federer remained in the competition, but only just, going five sets with Australia's John Millman, 4-6, 7-6 (2), 6-4, 4-6, 7-6 (8).

The Swiss master somehow came from 8-4 down in the match tiebreak, winning six consecutive points to claim his 100th win at the Australian Open and progress to the fourth round.

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Meanwhile, Greek hopeful Stefanos Tsitsipas was rocked by outside chance, Milos Raonic from Canada, with the No. six seed going down in straight sets, 7-5, 6-4, 7-6 (2).

Remaining unshakeable was Serbian Novak Djokovic, who beat his Japanese opponent, Yoshihito Nishioka, 6-3, 6-2, 6-2.

Equally as steadfast was Australian and women's world No. 1 Ash Barty, continuing her untroubled run into the final 16 by beating Elena Rybakina from the Ukraine, 6-3, 6-2.