Kei Nishikori feels he needs more time to reach full fitness after going down 6-3 3-6 6-4 to American Dennis Novikov in the opening round of a Newport Beach ATP Challenger Tour event in his first match since undergoing right wrist injury in August.
The former U.S. Open finalist, who last year reached the fourth round of the Australian Open in Melbourne, was forced to withdraw from the year's first grand slam and chose the second-tier event in California to launch his comeback.
"I had expected it to be tough and it was," Nishikori said.
"I'm not confident yet so I can't hit aggressive shots. It's going to take some time. I made mistakes where I shouldn't have, and there are a lot of gaps I need to fill. There were both good and bad."
Nishikori was ninth in the world rankings before suffering the injury ahead of the Cincinnati Masters last August.
He had been hoping to make his return at the season-opening Brisbane International, where he was runner-up in 2017, but withdrew from that event to continue his recovery.
The 28-year-old became the first Japanese man to reach a grand slam final at the 2014 U.S. Open but did not compete at Flushing Meadows last year because of his injury.
