World number one Roger Federer confirmed his fitness with a dazzling 6-3, 6-4 late night win over Lleyton Hewitt at Houston on Wednesday, providing a bright finish to a miserable rainy day at the Masters Cup.

Having not played since straining his left thigh muscle preparing for his home tournament in Basel a month ago, Federer's fitness was the big question hanging over the $4.4 million season finale.

A victory over Gaston Gaudio in his opening round-robin match eased some concern, but Federer's clinical dissection of his Australian rival in 75 minutes confirmed the 23-year-old Swiss was back playing the sublime tennis that carried him to three Grand Slam titles in 2004.

Behind a spectacular assortment of winners, Federer extended his domination over Hewitt and has now swept all five of their meetings this season, including a three set demolition of the Australian in the final of the U.S. Open.

"I played a good match, not as good as the U.S. Open but this is just my second match in a long time so you cannot expect to play as well as in the U.S. Open final," said Federer. "I just tried to play my game against him and for the moment that is enough. I'm just happy to be through."

After tornado warnings and torrential rain, play finally got underway after an eight-hour delay with Spain's Carlos Moya ending French Open champion Gaston Gaudio's hopes of a Masters title easing past the Argentine 6-3, 6-4.

By the time Federer and Hewitt made their way onto the floodlit centre court the clock was ticking towards midnight and neither player appeared particularly anxious to start the match.

But once the contest was underway, all reservations quickly disappeared as the pair engaged in the type of high-intensity battle expected when the world number one and defending champion clashes with a former number one and twice Masters Cup winner.

RAIN RETURNS

Federer seized control of the opening set by breaking Hewitt with a flowing backhand crosscourt winner for a 3-1 lead.

The Swiss also registered the early break in the second set and appeared headed to speedy with a 4-2 lead until rain again swept across the Westside Club.

An hour delay merely postponed the inevitable for Hewitt, who for all his effort could not find a chink in Federer's armour, and the Swiss closed out the match with an emphatic overhead smash.

"It's tough to come out after a rain delay down a set and a break against the best player in the world," said Hewitt. "I had a lot opportunities against him but I haven't taken them at the right time.

"Against the best players in the world you have to take them.

The win improved Federer record to 2-0 and guarantees him a spot in Saturday's semi-finals, while Hewitt can clinch a place in the final four with a win over Gaudio in his final round-robin match.