Fourth-seed Kei Nishikori retired in the first set of the Delray Beach International Tennis Championships on Tuesday due to a nagging left oblique injury.
Nishikori, coming off his third career victory at the U.S. National Indoor Championships in Memphis last week, was about to serve with 150th-ranked Ivo Karlovic of Croatia leading 5-4 in their first round match when he abandoned play.
Japan's best player, who rose six spots in the world rankings to 16th after his win in Memphis, called for a trainer to come out and work on his side after the seventh game.
"The pain was much worse than last week, and it was impossible to continue," said Nishikori, who won at Delray Beach for his first title on the ATP tour in 2008.