Marin Cilic crashed out of the US Open in the third round, losing 6-4 6-3 6-3 to Jack Sock at Louis Armstrong Stadium.
Cilic, the 2014 champion, was seeded seventh and entered the competition on the back of winning the Western and Southern Open in Cincinnati, where he beat world number two Andy Murray in the final.
However, the Croatian, who reached the semi-finals last year, was unable to maintain that form, losing in straight sets to 26th seed Sock.
Cilic is known for his formidable serve, but he produced one fewer ace than Sock's seven and got just 49 per cent of his first serves in.
The 27-year-old made 21 unforced errors and struggled to win points on his second serve, while Sock provided minimal opportunity for him to build momentum.
Sock, who celebrated by pretending to fence with his racket, faces another tough task in the fourth round - matching the furthest he has gone in any grand slam in his career - with Jo-Wilfried Tsonga next up.