The men's draw took a surprising turn for the second successive day, however, with second seed Rafael Nadal and number three Andy Roddick joining world number one Roger Federer on their way out of the tournament.

After losing to Nadal in last week's final at Indian Wells, Serb Novak Djokovic took quick revenge, shocking the Spaniard 6-3 6-4 to advance to the semi-finals without dropping set.

The 19-year-old Serbian will now meet Andy Murray, who reached the final four when Roddick was forced to retire with a hamstring injury trailing 5-3 in the first set.

After disposing of top seed Maria Sharapova in the fourth round, Williams made quick work of Vaidisova, overwhelming the 17-year-old in just 67 minutes to advance to the last four.

Playing her first tournament since lifting her eighth Grand Slam title at the Australian Open, Williams has gained form and confidence with every match, underlining her determination to return to the top of the world rankings.

Williams raced through the opening set but met resistance in the second when the eighth-seeded Vaidisova threatened to seize control, securing triple break point at 4-4.

DECISIVE BREAK

But the young Czech failed to capitalise and Williams immediately made her pay, recording the decisive break to go up 5-4 and winning 13 of the final 14 points to close out the match.

"I play tough when I'm down, I can always count on my serve," Williams told reporters.

Williams will next meet 14th seed Shahar Peer of Israel who booked her place in the final four with a 6-0 6-3 rout of Italy's Tathiana Garbin.

World number one Justine Henin of Belgium will take on Russian ninth seed Anna Chakvetadze in the other semi-final.

Murray, the 12th seed, was leading 4-3 when American Roddick called for a medical timeout to receive treatment in the locker-room.

The third seed returned to the court but was immediately broken. Roddick walked to the net to shake hands then informed the umpire he could not continue.

"I don't know what it is, I'm going to get an MRI, I think the medical term is, the bottom of my ass hurts," said Roddick, the 2004 Miami champion. "He hit a volley behind me and I lunged at it and hurt something.

"It's disappointing but right now I just want to focus on getting better."

The 19-year-old Murray moved into the semi-finals for the second successive Masters Series event having reached the last four at Indian Wells.

"I guess anybody would rather get off the court quicker than longer but I'd rather it was under normal circumstances," said Murray. "I'm definitely going to have enough time off to be 100 percent fit going into the match.

"I've lost my last two Masters semi-finals and I'd like to get to my first final."

Earlier in the week, Nadal ruled himself out of Spain's Davis Cup tie because of a sore foot but refused to use the nagging injury as an excuse for his loss.

Federer lost to Argentine Guillermo Canas on Tuesday.