Mardy Fish pulled out of the U.S. Open hours before his fourth-round match against Roger Federer on Monday, saying it was for "precautionary measures" on doctor's advice.
The 30-year-old American, who was seeded 23rd at Flushing Meadows, missed about 2½ months this season because of an accelerated heartbeat and had a medical procedure in May.
"We are not 100 percent sure what the issue is and if it is related to his previous issues," Fish's agent, John Tobias, wrote in an email to The Associated Press. "Mardy is fine and will return home to L.A. tomorrow. This was strictly precautionary and I anticipate that Mardy will play in Asia this fall."
Fish's third-round victory over Gilles Simon at the U.S. Open went five sets, lasting more than 3 hours and ending after 1 a.m. Sunday. Afterward, Fish did not attend a news conference; the tournament said he was getting medical treatment, but didn't elaborate.
In a statement released by the U.S. Tennis Association on Monday, Fish said he "was reluctant to" withdraw from the year's last Grand Slam tournament but was "following medical advisement." He added that he looks forward "to resuming my tournament schedule in the fall."
After losing a match at Key Biscayne, Fla., on March 29, Fish went to be checked by doctors because his heart started racing uncontrollably that night. He pulled out of the U.S. Davis Cup team's quarterfinal against France the following week.
In May, doctors induced extreme palpitations to try to pinpoint the problem in Fish's heart. He returned to the tour at Wimbledon in June.
The walkover allowed the top-seeded Federer to reach his 34th consecutive Grand Slam quarterfinal, extending his own record.
"I am really sorry for Mardy. I just want to wish him a speedy recovery," Federer said in a statement issued by the tournament. "We all want to see him back on tour soon."
Federer, who has won five of his record 17 Grand Slam titles at the U.S. Open, can reach the semifinals by getting past No. 6 Tomas Berdych of the Czech Republic, who eliminated No. 11 Nicolas Almagro of Spain 7-6 (4), 6-4, 6-1.
Berdych stunned Federer in the 2010 Wimbledon quarterfinals on the way to reaching the final there.
"It will be a tough match against Tomas. We have played many times in the past and he has always been a tough opponent," said Federer, who has won 11 of 15 career head-to-head matchups. "I will have to continue to serve well and dictate the points."
AP Sports Writer Rachel Cohen contributed to this report.