World No. 1 Ash Barty has been sensationally knocked out of Wimbledon after a three-set loss to American Alison Riske in their fourth round clash.

The Aussie tennis star's dream of winning back-to-back grand slams was ruined by the world No. 55 who came back from a set down to upset the top seed and bring a shock end to Barty's 15-match unbeaten run.

After Barty breezed through the first set in 32 minutes, Riske showed enormous fight — and no shortage of skill — to run her 23-year-old rival ragged as she found new levels of power in her ground strokes.

The unseeded US star seized on Barty's wavering serve and routinely found the corners of the court to send the Aussie into her first three-setter since the French Open semifinals in June.

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In a thrilling decider, both players slugged it out in long rallies before Riske found the decisive break, then served it out to seal the 3-6 6-2 6-3 victory.

Barty said she didn't execute like she needed to and revealed her biggest problem — the flaw that cost her the most — was her serve. She started in fine fashion but ended up finishing with a first serve percentage of just 51 per cent.

"Tough one. I think I started well. I was sticking to how I wanted to play," Barty said. "Then in the second set, I think my serve let me down. I let Alison get back into the match too many times, having looks at second serves.

"Overall I didn't play a poor match. When I needed to, when the big moments were there, Alison played better today. Tough one to swallow but I lost to a better player.

"Today at times when I really needed to back myself, the execution wasn't quite there.

"In the second set, I gave Alison too many looks at second serves. I know that my percentage was down. I wasn't missing serves by too much. They were small margins. A miss is still a miss, and gives her an opportunity to have a look at a second serve.

"Disappointing. That's probably one of the parts of my game today I'm disappointed with the most. But in saying that, it was just a bad serving day at times. Not always. There were times where it was still there, as well. But not to be.

"She (Riske) deserves to be in the quarter-finals. She played a great match today."

Riske was thrilled with the result and said she didn't want the tournament to end despite playing the World's No 1 player.

"You told me you weren't going to make me cry before we started this," Riske told ESPN's Tom Rinaldi in her post-match interview. "I told my parents I really don't want Wimbledon to end. Even if I got to the finals I wouldn't want it to end."

Barty was all business during week one at the All England Club, steamrolling her way into the fourth round with ease and her ruthless streak was on show to begin Manic Monday — one of the best days of the entire Grand Slam when all the remaining big names come out to play.

The Queenslander started with a bang, sending down four straight aces to take a 1-0 lead and make clear to Riske just what sort of challenge she was in for.

The highlight of the first set was Barty unleashing a cracking backhand slice winner down the line to give herself two break points, and she converted on the second opportunity to go up 4-1.

A sensational running forehand winner down the line made it 30-30 in the next game but Riske stayed composed and held, then jumped all over Barty's second serve to secure a break of her own only to waste her improved position.

Barty broke straight back when she unfurled a brilliant backhand winner down the line — a conventional two-hander this time rather than her one-handed slice — to make it 5-3 and the Aussie clinically served out the set.

Longer baseline rallies were a feature of the second set but Riske was always willing to come to the net and she broke for a 3-1 lead. The 29-year-old was working her angles brilliantly as she ran Barty all around the court and was brave enough to hit lines — a vast improvement on her opening set showing.

Up 4-2, Riske produced lovely sleight of hand to land a delicate drop shot out of Barty's reach and after winning the next point put herself in the frame to take the match to a decider with a 5-2 lead.

Sensing a real chance to upset the world's top ranked player, Riske pounced and broke again to level the match up at 3-6 6-2.

Barty's serving let her down badly in the second set. In the opener she won 93 per cent of points on her first serve but she wasn't giving herself enough of a chance to capitalise on that when her first serve percentage dropped to 46 per cent heading into the decider.

Compounding that, Riske's return game was on another level as she attacked Barty's serve with serious intent, and the American was enjoying more fortune on longer rallies.

The standard only rose in the third set. Even Barty had to nod her approval when Riske passed her with a punishing backhand winner to take a 2-1 lead and there were no easy points as both players fought like warriors to keep themselves in the contest.

The set stayed on serve, going to 3-3 but Barty would not win another game. Riske's intensity rose to new levels as she bullied Barty, hitting some monstrous ground strokes of both wings to claim one of the biggest wins of her career.