The 24th seed Muguruza stunned Williams in the second round at Roland Garros and another upset looked on the cards as the rising 21-year-old tore through the first set with a barrage of winners from the baseline.

Top seed Williams woke up and her serve, so often her saviour, became almost impenetrable.

After saving six break points in a marathon second game of the deciding set, Williams then broke Muguruza twice before sealing the match when her opponent blasted a forehand long.

Williams will play last year's losing finalist Dominika Cibulkova for a place in the semi-finals.

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