Japan's top-ranked men's tennis player Kei Nishikori said Monday a back injury will keep him on the sidelines for the rest of the 2021 season, but pledged to be back on court next year.

The 31-year-old former world No. 4 revealed on Twitter that he will pull out of the Nov. 7-13 Stockholm Open, the last event of the ATP season before the tour final, because of the injury he sustained in the weeks leading up to the San Diego Open in late September.

Nishikori withdrew with back trouble just 90 minutes before he was scheduled to play his San Diego opener against Britain's Andy Murray.