Djokovic retires from US Open with shoulder injury

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.