Defending champion Novak Djokovic reached the US Open last 16 as a second successive opponent pulled out through injury at Flushing Meadows.
The Serb, who had a walkover in the second round when Jiri Vesely conceded, was leading 4-2 when Russian Mikhail Youzhny quit with a leg problem.
"It's difficult to pick the words. I don't think I have had this situation before," said top seed Djokovic.
He will face Britain's Kyle Edmund or John Isner in the last 16 in New York.
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Djokovic, who has been troubled by a wrist injury in recent months, was on court for only 32 minutes.