After recovering from an upset stomach, the former world number one was out of form when he tried his best to earn a 7-6(4), 6-4 win over Stepanek, who stepped in to replace Andy Roddick.

Roddick pulled out of the tournament just hours before the match with Federer due to an ankle injury.

Federer reckoned that he would have not been able to play if the match had been scheduled one day earlier.

"I got sick, or started to feel really bad after my match was over with Gilles Simon (on Monday) only later on when I got back to the hotel, and shortly before I was going to bed," he said.

"Really upset stomach and (I) couldn't even practice. It would have been impossible to play yesterday," he said.

Federer, who lost his first match to Simon, is now in a precarious position as he has to beat in-form Murray in the final group round for a semi-final berth.

Murray qualified for the final four already with an easy 6-4, 6-2 victory over Frenchman Simon.

After beating Roddick in his opener, Murray's two-set win guaranteed his progression, despite him still having to play Federer.

Murray, the third seed, raced to a 4-0 lead and despite a fightback from the Eighth-seeded Frenchman, Murray eased to take the final set.

Despite Murray's strong start, a number of unforced errors allowed Simon back into the first set and the Scot was wobbling for a couple of games.

He held his nerve though and after surviving a number of crunching baseline rallies he took the opening set 6-4.

Things were a little more comfortable in the second set and with Simon looking increasingly agitated, Murray grabbed the win in dominant fashion.

Novak Djokovic became the first player to book his semis place on Tuesday after beating Nikolay Davydenko in three sets.