Last year's US open runner-up Ons Jabeur has been knocked out in the fourth round by China's Zheng Qinwen, who handed the ailing Tunisian fifth seed a 6-2 6-4 defeat.

An illness left Jabeur struggling to catch her breath at times earlier in the tournament and she ran out of puff against Zheng, who sent over 21 winners at Louis Armstrong Stadium to reach her first Grand Slam quarter-final.

Zheng will face the winner of the round four clash between 13th seed Daria Kasatkina and second seed Aryna Sabalenka.

"Right now I feel just super happy and excited to play in a big stadium and have a really good performance," Zheng said on court. "That was really an amazing win for me."

Jabeur follows fellow heavyweights, number three seed Jennifer Pegula and world number one Iga Swiatek, in exiting the tournament at the fourth round.

Pegula was stunned by Madison Keys 6-1, 6-3 in an all-American clash on Arthur Ashe Stadium.

Pegula beat the 17th seed Keys in their only previous meeting last year and was considered as one of the US's brightest hopes after winning the Montreal title last month. However, on Monday she lacked her usual firepower and finesse.

"I've had so many amazing moments in New York," said Keys, who is playing in her 12th US Open main draw.

"Being able to at any moment come back from any difficult positions I've been in matches has been amazing."

Meanwhile, defending champion Iga Swiatek exited the competition earlier this week and saw her reign as world number one come to an end after she unravelled 3-6, 6-3, 6-1 against Latvia's Jelena Ostapenko.

The Polish four-time Grand Slam winner had not dropped a set before Sunday's match but was unable to hold off the confident 20th seed's charge.

She will see Belarusian Aryna Sabalenka replace her as world number one when the standings are updated after the tournament. (Reuters)