Mardy Fish withdrew from the French Open with what organizers called fatigue, but the highest-ranking American player was actually suffering from a form of arrhythmia that required surgery.
Fish told USA Today on Sunday he underwent a procedure Wednesday in Los Angeles to correct faulty wiring in his heart.
"It felt like my heart was going to jump out of my chest," Fish said of the extreme palpitations that would wake him in the middle of the night.
"During days, I'm totally fine," Fish told USA Today. "I can track it and work out fine. But every time I would go to bed my mind would start racing. Is this going to happen tonight? Is this going to be another night like that? It was super hard to go to sleep."
Fish said the problem started in February before a Davis Cup match against Stanislas Wawrinka. The episodes happened a few more times in the weeks to follow, but the worst came following a quarterfinals loss to Juan Monaco at the Sony Ericsson Open in Key Biscayne, Fla.
Fish told USA Today he awoke around 3:30 a.m. with his heart beating three times faster than the expected rhythm for a world-class athlete at rest.
"I was completely panicking," Fish said of the 30-minute episode. "I thought I was going to die."
Fish woke his trainer and the two eventually decided to call 911. Fish left his hotel on a gurney that night and was taken to a local hospital.
Fish went to be checked by a doctor, then saw another doctor the next day. He pulled out of the U.S. Davis Cup team's quarterfinal against France the following week and the initial public diagnosis was fatigue, but Fish said doctors couldn't pinpoint the exact problem in his heart.
Fish then returned to training, using a small electrocardiogram unit to monitor his heart, according to the report. Fish tested the heart at a tournament in Houston on April 12, but Fish said he had another small incident in the early morning hours before his eventual opening-round loss.
"After Houston we really realized it was an issue instead of thinking it was a one-off," Fish told USA Today.
Fish said doctors rendered the problematic heart circuits "dormant" during Wednesday's surgery and he expects to recover fully.
"They feel like it was very successful, and that it's totally behind me now," Fish told the paper.
Fish says he plans to resume light training later this week and is aiming for a return at Wimbledon, or its grass-court tuneup at Queen's Club.
"I want to stress that I'm good now, I'm fine, they've fixed the problem," he told USA Today. "I don't want people to think I missed the French Open because I didn't want to go because I was just tired. That's just not the case.
"Look, I'm still in the top 10 in the world and still the No. 1 American. This is still a very fun time for me. I don't want to sit on my ass while I'm 10 in the world."
Information from The Associated Press contributed to this report.