Tenth-seeded American Frances Tiafoe was forced to retire Wednesday due to a lower-back injury down 6-4, 3-1 while facing Denmark’s Holger Rune, the No. 7 seed, at the Cincinnati Open in Mason, Ohio.

Tiafoe leapt out to an early lead in the fourth-round match, winning each of the first three games. But he proceeded to lose six of the next seven games, taking a medical break while trailing 4-5 in the first set, then retiring down a break in the second set.

Rune won 90 percent (19 of 21) of his first-serve points in the abbreviated match, finishing with more than twice as many winners (24 to 10) as Tiafoe.

“From 4-4, when I got the break, and at 5-4, when he had the medical timeout, that’s when I felt something was not right with him and after, obviously, I could see he was struggling a l

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