Roger Federer progressed to a US Open quarter-final against Tomas Berdych without hitting a ball on Monday as concerns about Mardy Fish's health resurfaced when he pulled out of their scheduled fourth-round match.
The American needed minor surgery earlier this year to correct an irregular heartbeat and admitted after his five-set second-round win over Nikolay Davydenko that doubts over his health remained.
Fish then had another long match against Gilles Simon in the third round, after which he did not have a press conference because of medical reasons.
The 30-year-old did not practice yesterday and, although no specific reason was given for his withdrawal, it is believed to be because of worries over his heart.
Fish issued a short statement, saying: "I regret that I have to withdraw from the US Open for precautionary measures. I was reluctant to do so, but am following medical advisement.
"I had a good summer and look forward to resuming my tournament schedule in the fall."
The walkover means Federer extended his record of consecutive grand slam quarter-finals to 34, and in the last eight on Wednesday he will play sixth seed Berdych.
The Czech has looked in fine form this tournament and reached his first quarter-final at Flushing Meadows with a dominant 7-6 (7/4) 6-4 6-1 victory over 11th seed Nicolas Almagro.
Federer said: "I am really sorry for Mardy. I just want to wish him a speedy recovery. We all want to see him back on tour soon.
"It will be a tough match against Tomas. We have played many times in the past and he has always been a tough opponent. I will have to continue to serve well and dictate the points."
Berdych and Federer have met 15 times before, with the Swiss player winning 11 of them, but Berdych did win their quarter-final clash at Wimbledon in 2011 before going on to reach his only grand slam final.
Although Berdych has now played four sets more than Federer in the tournament, he does not feel the walkover will have an effect when they meet in two days' time.
The Czech said: "It's at least good that I didn't stay that long on court, that I was able to save some energy as well.
"But we have a day off. I think it's going to be fine. Probably he doesn't care at all. Maybe if I would be in his situation, I would rather play a match and win it and keep the rhythm of one day match, one day off, and then play another one."
Berdych and Federer last met at the Masters Series tournament in Madrid in the spring, with the Swiss winning in three tough sets, but Berdych knows he is facing a different player in New York.
He said: "Our last six matches we played it's 3-3, so it's quite nice statistics. But this is another one. We didn't play for a couple of months.
"In that time he played some incredible tennis again. He won another grand slam, become number one again, so there is no better player at all to play right now.
"When I saw the draw in the beginning, that was the goal to get to play a match with Roger. I'm there, so I will try to do my best."
Also through to the quarter-finals is Croatia's 12th seed Marin Cilic, who ended the giant-killing run of Slovakia's Martin Klizan.
The match was moved to the Arthur Ashe Stadium in place of the Federer clash, and it was Cilic who came through 7-5 6-4 6-0, fighting back from a break down in the second set.
Cilic's progress has stalled since he reached the semi-finals at the Australian Open in 2010 as a 21-year-old but he looks to be getting back to his best form and will next meet either Andy Murray or Milos Raonic.
Klizan had beaten Jo-Wilfried Tsonga and Jeremy Chardy back to back and a cheque for $120,000 is sure to soften the blow of his exit.