Japan's Kei Nishikori suffered injury heartbreak at the US Open for the second year in succession on Monday when he retired from his first round clash against Italy's Flavio Cipolla. The 21-year-old Nishikori, who had been suffering from a back injury, called it quits after dropping the first two sets 6-4, 6-2. Nishikori said he had first felt the problem with his back at the US Open warm-up event at Winston-Salem last week where he played three qualifiers and three main draw matches. "It started Saturday. I practiced in the morning and started hurting, and I stopped practice and it got better yesterday a little bit," said the Florida-based player. "The beginning was okay today, but the pain was increasing every point, and I had to retire." Twelve months ago, Nishikori, now ranked at a career-high 46 in the world, was forced to retire from his third round tie against Spain's Albert Montanes with a groin injury. That had happened after he had defeated Croatia's Marin Cilic in a five-hour marathon in the previous round. "Last year was a great, great moment for me. I got a lot of confidence after beating Cilic," he said. "But it was just unfortunate that I got the injury. I didn't have much time from last week so that was the problem." Cipolla next faces either Alexandr Dolgopolov, the Ukrainian 22nd seed, who defeated Portugal's Frederico Gil, 6-4, 6-2, 7-5, for a place in the last 32. "I think I played a great first set. I couldn't tell he had a problem in the first set. I saw he wasn't moving 100 percent at maybe 2-1 in the second set. Then he started making a lot of mistakes," said Cipolla, the world 108. Cipolla made the third round here on his debut in 2008.