The world number one and top seed lost by two sets to one to Latvia’s Jelena Ostapenko.
Defending champion and world number one Iga Swiatek has been knocked out of the US Open in the fourth round.
Latvia’s Jelena Ostapenko, the 20th seed, beat Swiatek 3-6, 6-3, 6-1 at Arthur Ashe Stadium in New York on Sunday.
The Polish three-time Grand Slam winner had not dropped a set before the match against Ostapenko.
The Latvian fired off a crisp forehand winner to break her opponent’s serve on match point, pumping her fist as she extended her career head-to-head record against Swiatek to 4-0.
Ostapenko, who won at Roland Garros in 2017, next faces American sixth seed Coco Gauff, who beat wildcard Caroline Wozniacki in three sets earlier in the day.
Swiatek was also knocked out of Wimbledon early, after losing in the quarter-finals to unseeded Elina Svitolina from Ukraine.