The formerly unknown 24-year-old Frenchwoman who just beat arguably the greatest tennis player of all time admitted she was “scared” of her opponent.

Harmony Tan caused what will be one of the upsets of this year’s Wimbledon Championships when she knocked out 23-time grand slam winner Serena Williams on Wednesday morning (AEST) in a three-hour and 10 minute epic.

Watch the moment Harmony Tan defeated Serena Williams in the video above

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The world No.115 was making her first appearance in the main draw of tennis’s biggest stage, against 40-year-old Williams, who first competed at the Old England Club in 1998 – the year Tan was born.

Williams has since won seven Wimbledon titles from 11 finals, to go with her all time record of 23 grand slams.

Understandably, Tan was a bit lost for words, and even on the verge of tears, after her 7-5 1-6 7-6 (10-7) triumph.

“I don’t know what to say,” she said.

“I have watched [Williams] so many times on TV. For my first Wimbledon it is ‘wow’, just ‘wow’.

While she gave away her poker face post-match, you would never have seen it during her steely performance as she rose to the occasion against the tennis legend like few in the history of the game have been capable of.

Serena Williams frustrated with herself before going down. Credit: John Walton – PA Images/PA Images via Getty Images

“When I saw the draw I was really scared – Serena, she’s a legend,” Tan said.

“I was like, ‘oh my God! How can I play? I can win maybe one or two games.

“I play every ball like just, yeah, put some slice, some change, some variety on the game, and it works today.

“It’s a dream because, you know, I saw Serena on the TV when I was young. My coach, Nathalie Tauziat, played her 20 years ago.”

Tan revealed that even her coach didn’t sell her high expectations against the doyen of women’s tennis.

“[She told me to] enjoy because it’s your first time in Wimbledon. You play on Centre Court against Serena. Just enjoy the match,” Tan said.

She certainly did that, and spoiled Serena’s comeback party in the process.

The 40-year-old has battled injury for the past 12 months and had been effectively unsighted on the WTA tour until fans finally got to see the American make her return at the most famous tournament in the world.

Williams, who turns 41 in three months, had not played a singles match since injuring her hamstring at Wimbledon last year, only returning to doubles action earlier this month.

Harmony Tan’s elation after causing a boilover. Credit: Clive Brunskill/Getty Images

Following the defeat, Williams says she does not know what the future holds after her defeat, but the thought of playing in the US Open later this year still motivates her.

Sanguine, and maybe a little shell-shocked, Williams quipped wryly of her exit that “It is definitely better than last year.

“It definitely makes me want to hit the practice courts because when you’re playing not bad and you’re so close. I feel like that it’s ‘OK, Serena, you can do this if you want.’

“I think physically I did pretty good. I feel like in just those key points, winning some of those points, is something mentally that you have to have. I did pretty good on maybe one or two of ‘em, but obviously not enough.”

But Williams said she still has fire in her belly, especially looking ahead to Flushing Meadows.

“When you’re at home, especially in New York, and the US Open, that being the first place I’ve won a grand slam, is something that’s always super special. Your first time is always special,” she said”

“There’s definitely lots of motivation to get better and to play at home.

“I think if you’re playing week in, week out, or even every three weeks, every four weeks, there’s a little bit more match toughness.

“But today I gave all I could do. At some point you have to be able to be OK with that.”

Serena Williams was shattered. Credit: Rob Newell – CameraSport/CameraSport via Getty Images

Williams’ rustiness showed at the start when she lost the first two games, but won the next four.

Tan then broke back and held for 4-4, then broke again at 5-5 before serving out to claim the set.

World No.115 Tan had game point on Williams’s serve at the start of the second set, but the seven-times Wimbledon champion held and then won a 19 minute 41-second game to break.

She grabbed the momentum and ran away to take it 6-1.

To the delight of an adoring Centre Court crowd, Williams raced to a 3-1 lead in the decider. But a resilient Tan broke back and went up 4-3.

As the match clock ticked beyond three hours, inevitably the contest went into a tiebreak and Williams raced into a 4-0 lead.

But Tan simply would not go away, fighting back to lead 5-4 and winning it 10-7 with a rasping forehand.

Tan’s reward is a second-round clash with 32nd seed Sara Sorribes Tormo. The Spaniard, ranked world No.40, is yet to advance to round three in her two previous attempts.

With AAP

In the video below: Nick Kyrgios blows up at Wimbledon

Kyrgios blows up at Wimbledon.

Kyrgios blows up at Wimbledon.