(CNN) -- World No.1 Roger Federer progressed to his 34th consecutive Grand Slam quarterfinal without stepping on court on Monday after Mardy Fish withdrew from their last-16 match at the U.S. Open.
Fish, who underwent surgery to correct an irregular heartbeat in May, pulled out on the advice of doctors.
"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," the 30-year-old American said.
"I had a good summer and look forward to resuming my tournament schedule in the fall."
Federer, who remains on course for a sixth U.S. Open title, passed on his sympathies to the retiring Fish.
"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," Federer said.
Up next for the Swiss maestro will be Tomas Berdych who beat Spain's Nicolas Almago in straight sets 7-6 (7-4) 6-4 6-1 in two hours.
The sixth seed from the Czech Republic does not have a good record against Federer, losing 11 times in 15 meetings.
"I probably need to have something more than 100 percent," Berdych said.
Croatian 12th seed Marin Cilic is also through to the last eight after a comfortable straight sets (7-5 6-4 6-0) win over Martin Klizan from Slovakia.
Cilic with play the winner of the evening match between third seed Andy Murray and Canada's Milos Raonic.