Novak Djokovic has taken the initiative in a gruelling quarter final against Andy Murray at the US tennis Open in New York.

Djokovic leads two sets to one, taking the third set 6-2 after he and Murray traded tiebreak wins to win the first and second sets.

Serena Williams has fought back from an early deficit to keep her hopes of a sixth crown against Flavia Pennetta at Flushing Meadows alive.

The Italian was up 3-love early after two early breaks of serve, but Williams reeled off the next six games to take the set and cruised 6-2 in the second.

She says she didn't worry and knew she could claw her way back.

Williams will meet Ekaterina Makarova in the semi finals.