Defending champion Coco Gauff was knocked out of the US Open in the fourth round on Sunday by American compatriot Emma Navarro after a serving horror show.

The 13th-seeded Navarro took advantage of third-ranked Gauff's 19 double faults and 60 unforced errors to win 6-3, 4-6, 6-3 and set up a quarter-final against Spain's Paula Badosa.

"It's not the summer that I want," said Gauff who exited Wimbledon in the fourth round and suffered early losses at the Olympics, Toronto and Cincinnati.

"But I feel like there's 70 other players in the draw that would love to have the summer that I had even though it's probably my least well done during this time of the year," she added.

Gauff's defeat means that Serena Williams remains the last woman to successfully defend the US Open title back in 2014.

Navarro had defeated Gauff at Wimbledon in July and was dominant again on Sunday from the outset. (AFP)