World number one Simona Halep became the first top-seeded woman to lose in the US Open first round, falling to a 6-2 6-4 defeat against Kaia Kanepi.
The 26-year-old Romanian’s serve was broken five times in the first match on the new Louis Armstrong Stadium in Flushing Meadows, New York.
She fought back from a double break down to 4-4 in the second set but the Estonian broke again before Halep sent a forehand long on match point.
Halep also lost her opening match at Flushing Meadows last year.
The French Open champion was beaten by Maria Sharapova in 2017.
Before Monday’s defeat, no women’s top seed had lost in the opening round at the US Open since the professional era began in 1968, the same year that the tournament was given its current name, having previously been known as the US National Championships.
World number 44 Kanepi, 33, reached the quarter-finals at Flushing Meadows last year but has had a succession of injury problems in recent years.
Also on the opening day, British number one, Kyle Edmund, was knocked out in the first round of the US Open.
Italy’s Paolo Lorenzi beat 16th seed 4-6 6-4 7-5 6-1 amid gruelling humidity at Flushing Meadows.
After taking the opening set, Edmund, 23, was pegged back in the second before he cramped up in the third, affecting his movement on court.
The match, which lasted three hours 12 minutes, was played in temperatures of 30C and with humidity above 60%.