World number one Simona Halep was swept aside 6-2, 6-4 by 44th-ranked Kaia Kanepi on Monday, the first top-seeded woman ever to lose in the US Open first round.
It was a second straight first-round exit at Flushing Meadows for Romania's Halep, who dropped her opening match to Maria Sharapova last year.
The French Open champion had no answer for Kanepi's powerful groundstrokes.
The Estonian fired 26 winners to Halep's nine, and even her 28 unforced errors weren't enough to derail her challenge as she brought the rallies to quick ends and remorselessly punished Halep's second serve.
"Yesterday I thought I have beaten just once the world number one, today I had another chance," said a beaming Kanepi, who defeated then No. 1 Caroline Wozniacki in Tokyo back in 2011.
Kanepi, whose six Grand Slam quarter-final appearances include a run to the last eight in New York last year, earned a second-round meeting with Swiss qualifier Jil Teichmann, a 6-3, 6-0 winner over Dalila Jakupovic.
