Holger Rune is the first big name to fall on the men's side at the US Open.
The World No. 4 lost 6-3, 4-6, 6-3, 6-2 to Roberto Carballes Baena in his first-round matchup in New York.
It was the Spaniard's first career victory against a top-10 player.
"Disappointing day today," Rune told reporters postmatch, according to the ATP. "I had, as you know, a setback after the tournaments in Toronto and Cincy, where I had to retire. Not the best preparation I had.
"Not satisfied with the result, obviously, but also with my tennis. There is only one way, and it's to work."
Rune hasn't won a match since reaching the quarterfinals at Wimbledon last month. The 20-year-old's serve was broken seven times and he committed 43 unforced errors.
Carballes Baena will take on Aslan Karatsev in the next round.
