Ashleigh Barty's hopes of winning back-to-back grand-slam titles were ended by a shock fourth-round loss to unseeded American Alison Riske at Wimbledon.

World number one Barty had won 15 matches in a row and had not so much as dropped a set since the French Open semi-finals a month ago but Riske played superbly and the 3-6 6-2 6-3 victory was the biggest of her career.

The 29-year-old, who has also knocked out Donna Vekic and Belinda Bencic in a giant-killing run, is through to the quarter-finals of a slam for the first time, where she could face Serena Williams.

Naomi Osaka was overwhelmed by the pressure of being world number one for every one of her 21 weeks at the top, yet Barty had made it look like a natural progression.

She is only two years older than her Japanese rival but has much more life experience having been consumed by huge expectations as a young teenager, stepped away from the game for the best part of two years and then returned in 2016.

At her comeback event in Eastbourne three years ago, Barty's run was ended in the semi-finals by Riske, who is the closest thing to a grass-court specialist in the game these days.

The 55th-ranked American – whose first 13 tour-level match wins all came on grass – arrived at the All England Club with two trophies in her bag and was enjoying her best run at Wimbledon.

Barty began the match with four straight aces and hit 12 in all yet one of her biggest weapons was also an Achilles heel, with the Australian struggling to land her first serve.

In the second set her first-serve percentage was down at 40 and she paid the price, with Riske's flat yet precise ground strokes causing Barty all sorts of problems.

For the first time the 23-year-old looked a little flustered, and she had to save two break points to hold for 3-3 in the decider.

The pressure told in her next service game, though, as Riske sent yet another winner down the line to break, and a final backhand wide from Barty brought the curtain down on her campaign.

Barty's loss means Karolina Pliskova can overtake her to become world number one by reaching the final.