American Jack Sock sent Marin Cilic crashing out of the U.S. Open in the third round Friday, defeating the 2014 champion, 6-4, 6-3, 6-3, at Louis Armstrong Stadium.
Cilic was seeded seventh and entered the competition on the back of winning the Western and Southern Open in Cincinnati, where he beat world No. 2 Andy Murray in the final.
However, the Croatian, who reached the semifinals last year at Flushing Meadows, was unable to maintain that form, losing in straight sets to the 26th-seeded Sock.
Cilic is known for his formidable serve, but he produced one fewer ace than Sock's seven and got just 49 percent 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.