Mardy Fish eased his way back into big-time tennis with an exhibition match against top-ranked Novak Djokovic. The real test comes this week in the Southern California desert.

The 32nd-ranked American returns to the ATP Tour after a six-month absence caused by a heart condition Fish has yet to fully reveal.

It will be a cautious first step back into the whirlwind of travel, late-night matches, erratic eating and pressure. Fish, a former top-10 player, has been off the tour since September and has changed his lifestyle significantly — healthy eating and bedtime before 10 p.m.

But he was up late Monday night after losing to Djokovic in a tiebreaker of their eight-game pro set at the Los Angeles Tennis Challenge, an exhibition Fish put together with pal and former pro Justin Gimelstob.

"I don't feel 100 percent but I'm getting pretty close," the 31-year-old Fish said from a massage table as he was put through various contortions by his physical therapist. "We were playing pretty quick. I was getting winded. You can't duplicate that in practice."

"He looked quite good on court," Djokovic said. "He was moving well and serving well."

Fish will find out exactly how well his game holds up at the BNP Paribas Open in Indian Wells, Calif., a two-week hard-court event featuring the world's top players. He's been idle since withdrawing from the U.S. Open in early September on a doctor's advice, hours before he was to face Roger Federer in the fourth round.

Fish's health woes first surfaced a year ago this month, when his heart started racing uncontrollably after a match at Key Biscayne, Fla. In May, doctors induced extreme palpitations to try to pinpoint the problem in Fish's heart. He briefly returned to action at Wimbledon in June.

Fish has kept the exact nature of his problem private.

"I've gone back and forth whether to spill it all or keep it in," he said. "What I went through is the toughest thing I ever had to deal with in my life. It's not something that's very easy to talk about."

Even with friends. Fish said he wouldn't return their texts asking what was wrong, nor would he discuss details.

"It took me months and months and months to have a glass of wine at dinner, to go out to a movie with my wife," he said. He and Stacey have been married 4 ½ years.

"She was an angel," he said. "I've slept in the same bed with her every single night since April. I'm not sure where I would be without her help and her calming presence next to me in the bad times."

Fish said he visited the Mayo Clinic "to make sure everything is working properly and it is."

For three months, Fish didn't leave the house, work out or play tennis. One benefit to laying low was that the time off cured the tendinitis in his right arm that had plagued him for three years.

"I've retired 15 times in my head," he said. "For the first three or four months, I was done for sure. Then gradually I started feeling better and working out. You start missing tennis and the guys."

He added: "I haven't felt better than this since the U.S. Open."

Fish is planning to resume a full schedule of tournaments. He believes his health situation is "always going to be a part of me. I just want to be normal again."

Fear of the unknown plays a part in his concerns.

"It's something that's very uncomfortable," he said. "It's sort of fear of what might happen. It's sort of taught me how to handle stress better."

Fish has six career singles titles and reached a No. 7 ranking in August 2011. Four more singles victories will give him 300 for his career.

"I still feel like I can play at a high level for many years," he said.