Fish has played only eight matches on the ATP World Tour in 2013, going 3-5. He skipped all of the Grand Slam tournaments this season while entering only five other events on the main tour.
A year ago, Fish reached the fourth round at the U.S. Open but withdrew before he was to face Roger Federer at that stage, saying it was for "precautionary measures" on doctor's orders. Fish missed about 2½ months earlier last season because of an accelerated heartbeat.
The night after he lost a match at Key Biscayne, Fla., in March 2012, Fish's heart started racing uncontrollably. That May, doctors induced extreme palpitations to try to pinpoint the problem. He returned for last year's Wimbledon.
During the hard-court tournament in Washington earlier this month, Fish spoke about the difficulties he's faced the last two seasons.
"I struggled mightily during the U.S. Open last year. So those sort of post-traumatic experiences that I've had that I have now are what I need to get over right now," he said. "But it's a process. It's not an easy game. And fitness is as big a part of it as playing, and sometimes that's trouble spells trouble for me."
Asked then whether his heart or mind is the source of greater problems, Fish replied: "It's a little bit of both. A lot of it's mental, what I went through, and obviously a little bit of it was physical, too. I can't say it any differently than: It's just going to be a day-to-day thing right now."
Fish, who was born in Minnesota and now lives in Los Angeles, reached a career-best ranking of No. 7 in 2011, and is 173rd this week.
He is a three-time Grand Slam quarterfinalist once each at the U.S. Open, Wimbledon and the Australian Open, most recently at Flushing Meadows in 2011.