Top seed Roger Federer was left hoping for "a miracle" on Wednesday after being laid low by illness as he struggled to stay in the hunt for the Masters Cup title.
Despite beating first alternate Radek Stepanek 7-6, 6-4 in the Red Group on Wednesday, the world number two's defeat by Gilles Simon on Monday means he must now beat in-form Andy Murray to avoid an end to his quest for a fifth Masters Cup.
World number four Murray, who has a 3-2 career advantage over Federer, has enjoyed his best year on the circuit and although he has already qualified for the last four the Briton said he would be going all out for a win on Friday.
"I guess I got really lucky my match was scheduled late today and then Radek was not playing with his own racket, so that made it a little bit more lucky," the Swiss player told reporters.
"At the end of the day I still have a chance. I hope with a day of recovery I'll make a miracle happen here and get through to the semis."
Already short of practice, the last thing Federer needed was the illness that struck him after the defeat by Simon.
"I woke up in the morning and I was feeling terrible," he said. "Really upset stomach and couldn't even practice.
"I didn't go out of the room. I'm just sort of trying to get over things. It would have been impossible to play yesterday."
Federer has had a disappointing season by his high standards and ill health has played its part.
He started the year suffering from mononucleosis and two weeks ago pulled out of the Paris Masters with a bad back.
The 27-year-old Federer has never gone out before the knockout stage at six previous Masters Cups.
Czech Stepanek only came into the event as an 11th-hour replacement for injured American Andy Roddick.