By Mark Orlovac BBC Sport at Wimbledon

Roddick pounded down some huge serves on his way to victory

Number two seed Andy Roddick continued his charge with a straight-sets victory over a stubborn Sjeng Schalken.

The American was forced to dig deep against the Dutchman before reaching the semi-finals with a 7-6 (7-4) 7-6 (11-9) 6-3 victory.

Schalken pushed Roddick all the way and had three set points in the second but failed to convert the one on his serve.

Roddick also saved two break points at the start of the third and claimed the crucial break with a forehand winner.

The number two seed admitted he was relieved to edge out a determined Schalken to reach the last four for the second year running.

"It was a dogfight, Sjeng doesn't look spectacular, but he's a great player," said Roddick.

I don't see a weakness in his game - it's solid Sjeng Schalken on Andy Roddick

"If I left a return anywhere close to being short, he was striking it great to the corners but the rain breaks helped me, each time we came back I played better."

The 21-year-old now faces Tim Henman's conqueror Mario Ancic for a place in the final and he is ready for a stern test.

"I played Ancic at Queen's and I was lucky to win to be honest so I know he can play," said Roddick.

"Mario definitely has the tools to develop into a consistent contender. He fights every match."

Schalken admitted that Roddick's booming serve was key to the victory on Court One.

Roddick fired the quickest serve ever seen at Wimbledon, 146mph, and had an average first-serve speed of 132mph.

"Andy's serve has a lot of power and you just don't have any time to react," said Schalken.

"I don't see a weakness in his game, he is only going for it on his serve and his game is solid."