NEW YORK — Roger Federer will not win a 21st Grand Slam event singles title this year.

Hampered by tightness that developed in his upper back and neck Tuesday afternoon, a noticeably slow Federer was stunned by world No.78 Grigor Dimitrov of Bulgaria in the U.S. Open quarterfinals, fading as Dimitrov pulled off a 3-6, 6-4, 3-6, 6-4, 6-2 stunner. Federer committed a staggering 61 unforced errors against Dimitrov, who will face Daniil Medvedev of Russia in the semifinals.

Dimitrov, 28, was known as “Baby Federer” early in his career because he also deploys a one-handed backhand. On Tuesday, he was himself, not a miniature Federer, though he still admires the Swiss master. “He’s a great player, plays amazing, does amazing shots, serve, everything,” Dimitrov said. “There’s not enough words to describe him clearly.”

Federer, 38, wasn’t near that level Tuesday. He took a medical timeout after the fourth set and left the court to get treatment, but it didn’t seem to help. “Just needed to try to loosen it up, crack it and see if it was going to be better,” he said.