Serena Williams branded herself "unprofessional" after her bid for a record-equalling 24th grand slam singles title at the Australian Open ended with a shock third-round loss to China's Wang Qiang.

When they met at the US Open last summer, Wang won just one game and 15 points, but it was a completely different story this time as the 27th seed claimed a 6-4 6-7 (2) 7-5 victory.

Williams won her first tournament in three years in Auckland earlier this month and arrived in Melbourne with a determined intensity that seemed to indicate she was ready to put four recent grand slam final losses behind her.

It would have been fitting, meanwhile, had the 38-year-old finally moved level with Margaret Court in Melbourne as the tournament marks the 50th anniversary of her calendar Grand Slam.

But Williams made 56 unforced errors compared to 20 for her opponent, and said: "I just made far too many errors to be a professional athlete today.

"If we were just honest with ourselves, it's all on my shoulders. I lost that match. It's not about the tournament, it's just like I can't play like that. I literally can't do that again. That's unprofessional. It's not cool."

Wang is a much better player than she showed in New York but few would have given the 28-year-old much of a chance of reversing the result.

But, the more Williams was unable to pull away from Wang, the more the tension in her body grew.

Wang was on the brink serving for the match at 5-4 in the second set only for Williams' champions' mentality to kick in.

When she won the tie-break, it seemed tennis had read this script before and Williams would pull away to win, but instead Wang refused to be intimidated and kept her nose in front.

The Chinese player's nerves were obvious as she flunked her first two match points but Williams simply could not cut out the errors and a netted backhand sent her crashing out of the tournament.

The American admitted she thought she had done the hard work by winning the second set, saying: "I was optimistic I would be able to win. I thought, 'OK now finish this off'. I honestly didn't think I was going to lose that match."

Asked if defeats still sting as badly as they used to now she has so many other things in her life, Williams said: "I am just a better actress. I'm no happier than I was 10 years ago. But I just have to pretend I don't want to punch the wall, but in reality I do."

It was her earliest defeat at a hard-court grand slam since here in 2006, and will add fuel to those who believe a 24th title will remain elusive.

Williams is not one of those, and she will hold high hopes for Wimbledon and the US Open having reached the final at both events in 2018 and 2019.

"I definitely do believe or I wouldn't be on tour," she said. "I don't play just to have fun. To lose is really not fun. I seem to do well the last two slams of the year. I have won them all several times. Each one is definitely an opportunity for me to go out there and win."

While the focus was on Williams, this was a special day for Wang, particularly given what happened in New York last summer.

"I'm really happy now," she said. "I always believed I can do this one day. I didn't know which day, but it's coming today. I did really hard work in the off-season, so it paid off."

Wang was coached for four years by Australian former doubles great Peter McNamara, who died from cancer last summer.

"I always dream about him," she said. "I think he can see what I play today. He will proud of me. I miss him."

In the fourth round, Wang will face unseeded Tunisian Ons Jabeur, who ended the career of Williams' great friend Caroline Wozniacki.