Shanghai - Holder Roger Federer kick-started a late run at the Masters Cup on Wednesday by beating alternate Radek Stepanek 7-6 (7-4), 6-4 for his first Red group victory.

The Swiss, who has lifted the year-end title at four of the last five editions, stands 1-1 in round-robin play as rival Andy Murray grabbed a semi-final spot at the elite season wrapup.

The Scot rolled into the last four during his debut on the year-end big stage, defeating Frenchman Gilles Simon 6-4, 6-2.

Murray polished his record to a perfect 2-0 as he stopped the last player to enter the eight-man field. Simon gained a place at the last minute when world number one Rafael Nadal pulled out with a knee injury which will require a month or more to fully heal.

'Simon obviously is playing the best tennis of his life and he's in the top 10," said Murray.

"To beat a couple of top 10 players back to back is great. The Masters Cup is obviously a huge, huge competition for all the top players. I've got a chance of winning it now."

Two days after being beaten by Simon, Federer had his struggles to beat Stepanek, who stepped in as Andy Roddick withdrew with a turned ankle.

Since losing to Stepanek in Rome, Federer has beaten the Czech at the US Open and Madrid. The Swiss stayed alive as his opponent double-faulted on match point.

Federer will bid to reach the weekend on Friday when he plays his last group match against Murray, a re-run of the US Open final won by the Swiss. Simon, like Federer on 1-1, plays Stepanek.

Federer has a huge reputation to live up to at the season-ender as he makes a seventh consecutive appearance with a 27-4 record. He has reached the last five finals.

Stepanek got his chance to increase his 50,000-dollar payday just for fronting up as Roddick withdrew just hours before the match.

"In practise I just went over on it, we tried as much as we could to get it better," said the American. "I can't really stop and start very quickly. It's anything when I go kind of over on the outside it hurts.

"I do a lot of that a lot on my serve also, so it's definitely a tough prospect trying to beat Roger with no serve and not being able to move much.

"It's unfortunate for this event, but I don't think we're looking at anything more than a week or so," Roddick said.

Murray took a 4-0 lead in the first set against Simon, and had two points for 5-0 before the Frenchman went into recovery mode.

"Today I felt perfect," said the Scot. "I could have gone three, four hours if I had to. That gave me a lot of confidence.

"I was not doing as much running as him."