Britain's Johanna Konta suffered a shock first-round defeat against unseeded Serb Aleksandra Krunic at the US Open.
Konta, seeded seventh, was among the favourites for the title and one of eight women who could have ended the tournament as world number one.
However, a first-serve percentage of just 45% allowed the clean-hitting Krunic to attack off the return, and the world number 78 fought back for a 4-6 6-3 6-4 victory.
Britons Kyle Edmund and Cameron Norrie had earlier claimed first-round victories in the men's singles, but Heather Watson was beaten in the women's singles.
Coming two days after Andy Murray's withdrawal with a hip injury, Konta's defeat means neither British number one player survived beyond the first afternoon.
Asked how she would deal with the defeat, she said: "I think not to catastrophise is important.
"It is a tennis match. It's a sport. I think to have a healthy perspective on that, in general, goes a long way."
Konta ends Grand Slam year in disappointment
Konta, 26, looked in command when she moved 4-1 clear in the opening set on a quiet Grandstand Court that had just seen home hope Steve Johnson win.
Within an hour, the stands had begun to fill and the atmosphere intensified as Krunic began to seriously threaten an unconvincing Konta.
The Briton's first serve deserted her, slipping from 41% in the first set to a desperate 39% in the second.
Krunic dominated the final three games of the second to level at one set all and looked the more likely as Konta grimly clung on in the third.
The seventh seed screamed "come on" after holding in a lengthy service game for 3-2, but Krunic would break at the next opportunity and serve out the match brilliantly.
Konta sent a backhand wide on match point and was off the court within seconds, ending her Grand Slam year in disappointment.
"I don't think I necessarily played my best tennis, but my opponent also had something to say about that," she said.
"She played very freely, I felt, and she moved incredibly well. She made it very tough for me to be able to get any easier points.
"I don't take anything for granted. I think it would be quite obnoxious of me to come in here expecting I have a right to be in the second week."
I was not a hard worker - Krunic
Krunic last made waves at a Grand Slam when she came through qualifying to reach the fourth round at the 2014 US Open.
Her third-round win over then world number four Petra Kvitova was her last top-10 victory - until now.
At 5ft 4in tall, the Serb relied on the same speed of foot against Konta to set up 18 forehand winners, while her total of 36 unforced errors was six short of her surprisingly wayward opponent.
"Three years ago everything worked out well in that week," said Krunic, who faces Ajla Tomljanovic in the second round.
"Before and after that I was not a hard worker, this year I have been working really hard and I try and cool myself down on the court - I am not sure if it is obvious, it is hard.
"I am now winning more matches and that is what matters."
'Konta's biggest disappointment in two years' - analysis
Russell Fuller, BBC tennis correspondent at Flushing Meadows
This is the biggest disappointment Konta has endured since her breakthrough Grand Slam run here in New York two years ago.
Her serving was a long way from its best, but it was also the pressure exerted by Krunic which contributed to Konta's erratic performance.
The Serb went for her shots, and was aggressive at times despite an obvious lack of power. She also varied the pace very cleverly and dragged Konta in towards the net - and out of her comfort zone.
The next target for Konta is the end-of-season WTA Championships in Singapore. She may find herself outside the eight qualifying positions by the end of the US Open, so the lucrative WTA events in Wuhan and Beijing in the autumn are likely to prove decisive.