Kei Nishikori has retired from his first-round match at the U.S. Open because of a lower back injury.

Flavio Cipolla was leading 6-4, 6-2 on Monday when Nishikori retired.

A year after he won a match that took nearly five hours, Nishikori lasted just 75 minutes at the 2011 tournament. Nishikori thinks the injury was the result of fatigue after playing six matches at Winston-Salem last week, where he qualified and reached the round of 16.

Nishikori needed 4 hours, 59 minutes to upset 11th-seeded Marin Cilic 5-7, 7-6 (6), 3-6, 7-6 (3), 6-1 in last year's second round. Two days later, he retired with a groin injury during the second set against Albert Montanes.

The 21-year-old Japanese player is ranked 46th, matching his career best.