Johanna Konta became the first major casualty at the US Open as the seventh seed suffered a three-set defeat to Aleksandra Krunic in round one at Flushing Meadows.
Konta was one of eight players who began the year's final grand slam with a chance of claiming the world number one ranking.
However, the Briton's slim hopes of taking top spot were immediately dashed as world number 78 Krunic - a giant-killer in New York three years ago - came from a set down to triumph 4-6 6-4 6-3.
After coming through qualifying in the 2014 US Open, Krunic beat Madison Keys and third seed Petra Kvitova before also winning the first set in an eventual fourth-round defeat to Victoria Azarenka.
Krunic again showed her ability to thrive against a more illustrious opponent on Monday, although the Russian was aided by an error-strewn display from Konta, who landed only 45 per cent of her first serves.
Konta did recover one break in the final set, but soon dropped serve again to give Krunic a 4-3 lead.
And the underdog showed no signs of nerves serving out the match, starting the game with an ace before a backhand error from Konta brought proceedings to an end.
Krunic will face Ajla Tomljanovic or Johanna Larsson in round two.