His attacking style had given Murray problems from the start and, after saving four break points in game two, he broke serve in the following game and moved impressively to 5-2 with six aces and 20 winners.

BBC: Stanislas Wawrinka beats Andy Murray at US Open

Top-seeded Petrova hit seven aces in beating Zheng Jie of China 6-2 6-3 in the third round in Florida.

BBC: SPORT | Tennis | Petrova and Williams both advance

And Andy Roddick took less than an hour to dispatch Juan Carlos Ferrero 6-2 6-4, serving 18 aces and conceding just 11 points on serve.

BBC: SPORT | Tennis | Nadal struggles to win in Dubai