Rafael Nadal muscled his way into the quarter-finals with a 6-4, 6-2 win over Igor Andreev and then revealed the difficulties he had endured earlier in the competition. The Wimbledon and French Open champion said in his victories against Potito Starace and Lleyton Hewitt, he had found the ball "very fat".

The Spaniard explained that due to the temperatures and humidity, he had found the ball heavier than he was used too.

He said: "It was okay today, I got used to it. The ball seemed lighter and this made my game from the baseline easier. I am playing very well now and happy with my game."

After a tough opening set against Russia's Andreev, the world No1-elect went on to clinch the second with ease.

During the match, Nadal found the antics of the crowd frustrating when they burst into round of clapping or failed to take their seats quickly after end-changes. He made clear gestures to demonstrate his annoyance.

He now meets Jurgen Melzer, who put out Yen-Hsun Lu, the player from Chinese Taipei who eliminated Britain's No1 Andy Murray in the first round. There was no chance of a shock today, though, as the Austrian eased through 6-2, 6-4.

Nadal said he expected Melzer to be an "aggressive" opponent who would try and follow his serve into into the net as often as he could.

Roger Federer, who Nadal beat at Wimbledon, is through after beating Tomas Berdych of the Cezch Republic 6-3, 7-6.

Third seed and Australia Open champion Novak Djokovic is also in the last eight thanks to a 7-6, 6-3 win over Russia's Mikhail Youzhny.

In the women's singles, China's hopes of medal contention were lifted as the country's No2 Na Li beat Estonia's Kaia Kanepi 4-6, 6-2, 6-0 in the third round.

Li was roared on by vociferous home support desperate for a Chinese player to reach the latter stages of a competition which has been left wide open by a number of high-profile pull-outs.

She said: "I wanted to win but I played conservatively and I couldn't find my rhythm at first. I was completely unfamiliar with her game and she is a good player who reached the quarter-finals of the French Open. But finally I got my chance and took it."

Li will meet either Venus Williams or Victoria Azarenka of Belarus in the quarterfinals. She added: "I just want to have a good rest and don't want to worry about the next match yet."