Novak Djokovic has played only six games for two victories at the US Open after Mikhail Youzhny retired from their third-round match with a leg injury.
Djokovic was also handed a walkover in round two on Wednesday, when Jiri Vesely pulled out before a ball was hit - and the Serb only had to lead 4-2 before Youzhny followed suit.
The withdrawals have come as a timely boost for Djokovic, who has been struggling with both wrist and arm injuries at Flushing Meadows.
The world number one last came through a full match on Monday and he will have enjoyed almost five days rest before facing either Britain's Kyle Edmund or American John Isner in the last 16.
"I don't think I ever had this situation in my career," Djokovic said on court afterwards.
"Obviously I wish Mikhail a speedy recovery. I think he carried the injury into this match, that's what he said.
"I'll try to get the positives out of these six games. I did start with good intensity so I'll try to carry this into the next round."
