French Open runner-up Dominic Thiem suffered an opening-round defeat at the US Open as Thomas Fabbiano claimed a 6-4 3-6 6-3 6-2 victory at Flushing Meadows.
Having overcome Stefanos Tsitsipas in the first round at Wimbledon, world number 87 Fabbiano repeated the feat of knocking out a seed at the first hurdle in the final grand slam of the year.
Thiem, who was also beaten in the first round at Wimbledon but bounced back by winning in his home tournament at Kitzbuhel, was never allowed to get into his stride by the Italian, who earned his first top-five win.
Fabbiano struck 12 fewer winners than the world number four, but Thiem's usually reliable backhand proved his undoing, as the 25-year-old made a total of 48 unforced errors.
The defeat means Thiem's grand slam season ends in disappointment following his superb run at Roland Garros, though the clay-court specialist can console himself with three ATP titles in 2019, and an equal career-high ranking.
Fabbiano, meanwhile, will face Alexander Bublik in round two, after the Kazakh's defeat of Santiago Giraldo.