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.
