Serena Williams: 'I had to play my best match'

Serena Williams has said that she needed to play her "best match" of the Australian Open so far to book her place in the quarter-finals of the tournament.

The world number one had to come from a set down to beat Garbine Muguruza, who knocked her out at the French Open last year.

The American told reporters: "I had to play the best match of the tournament or else I was going to be out. When I have to go up a level, I have to. I can't afford to stay at the same level or I will be where I was at the French Open."

Williams will face last year's beaten finalist Dominika Cibulkova in the last eight.