Belinda Bencic knocked out defending champion Naomi Osaka ©Getty Images

Reigning women's champion Naomi Osaka was knocked out in the fourth round of the US Open after coming unstuck to a superb performance by Switzerland's Belinda Bencic.

The Japanese top seed, who was defending her ranking points from her victory at Flushing Meadows last year, will lose her world number one spot after her defeat in New York.

Bencic, seeded 13th, recorded a 7-5, 6-4 success to reach the US Open quarter-finals for the second time, matching 2014.

The 22-year-old raced into a 3-0 lead after an early break in the opening set but Osaka got back on terms.

In the 11th game, Bencic broke again and served out the set.

Osaka, who has also been beaten by Bencic at Indian Wells and Madrid this season, later claimed she was struggling with a knee injury.

The 21-year-old double-faulted at a key time in the second set to hand the Swiss another break, and Bencic served out the match to love.

"The challenge cannot be bigger than Naomi," said Bencic.

"I had to be at the top of my game. 

"I'm pleased how I managed my nerves.

"I had to take her serve early, try to anticipate, because she has a lot of power.

"I tried to play it a little bit like chess and tried to anticipate on the court."

Australia's Ashleigh Barty will now return to world number one, despite her US Open campaign also ending at the fourth round stage yesterday.

The French Open champion only surrendered top spot to Osaka last month.

Bencic will play Donna Vekić of Croatia in the quarter-finals after the 23rd seed recovered from the brink of defeat to knock out Julia Görges, 6-7, 7-5, 6-3.

The German served for the match at 7-6, 5-4 but became error-prone and served three double faults.

Görges, the 26th seed, did have a match point but she stroked into the net and Vekić turned things round.

Spain's Rafael Nadal came through in four sets in a battle of former champions against Croatian Marin Čilić ©Getty Images
Spain's Rafael Nadal came through in four sets in a battle of former champions against Croatian Marin Čilić ©Getty Images

Belgium's 25th seed Elise Mertens was comfortable in her progression, ending the run of American wildcard Kristie Ahn, 6-1, 6-1.

Another home player was also knocked out as wildcard Taylor Townsend's superb tournament was halted by Canada's 15th seed Bianca Andreescu, 6-1, 4-6, 6-2.

Mertens will now meet Andreescu in the last eight.

In the men's draw, Spain's Rafael Nadal knocked out Croatia's Marin Čilić in a fourth-round battle between two former Flushing Meadows winners.

Second seed Nadal, bidding for a 19th Grand Slam title and fourth US Open, came through 6-3, 3-6, 6-1, 6-2 against the 22nd seed who won the tournament in 2014.

"For some moments in the second set I felt there were too many points in his hands, he was pushing me back, he had more aggression than me and he hit every ball very strong," said Nadal.

"After that second set I thought something needed to change or I would be in his hands. 

"I started to return the second serve inside the court and that created a different perspective."

Nadal will meet Argentina's 20th seed Diego Schwartzman in the last eight after his 3-6, 6-2, 6-4, 6-3 win over Germany's sixth seed and ATP Finals champion Alexander Zverev.

Italy's 24th seed Matteo Berrettini is also through after a 6-1, 6-4, 7-6 win over Russia's Andrey Rublev and will play France's 13th seed Gaël Monfils, a 6-1, 6-2, 6-2 winner over Spain's Pablo Andújar.

The quarter-finals begin tomorrow at the hard court tournament, the last Grand Slam of the year.

Switzerland's 20-time Grand Slam champion Roger Federer will continue his quest for an open-era record sixth US Open against Bulgaria's Grigor Dimitrov.

American Serena Williams, a 23-time Grand Slam winner and six-time US Open champion, meets China's Wang Qiang.