Novak Djokovic has retired from the US Open with a shoulder injury.
The defending champion had lost the first two sets in his round of 16 match to Stan Wawrinka before pulling out.
The result takes Wawrinka into the quarter-finals of the tournament as he looks for his fourth Grand Slam title.
Meanwhile, Roger Federer eased into the final eight with a straight-set win over David Goffin in just 78 minutes.
Elsewhere, Serena Williams needed medical attention for her ankle as she overcame Petra Martic to reach the quarter-finals for the 16th time.
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