Britain's Andy Murray downed Gilles Simon in straight sets to reach the Masters Cup semi-finals on Wednesday as Andy Roddick joined Rafael Nadal on the injury list.
  
Murray overcame some dogged resistance to win the first set and ran away with the second to take it 6-4, 6-2 and follow third-ranked Novak Djokovic into the last four.
  
Murray beat Simon in Hamburg and in the Madrid Masters final this year, both in straight sets, and victory looked assured as he raced to a 4-0 lead in the first set.
  
But his charge was suddenly slowed by Simon, who stunned top seed Roger Federer in his opening match. The Scot was broken at 4-1 and he faced more break points before serving out the set 6-4.
  
Murray was again in trouble early in the second but he saved three break points and then broke twice for a straightforward win.
  
Meanwhile Roddick was a last-minute withdrawal from his match with defending champion Federer after turning his right ankle in training a day earlier.
  
"I came out here and tried to warm up 45 minutes or so ago, and it was pretty apparent that my movement was probably 30 or 40 percent," he told reporters.
  
"Unfortunately that's just not good enough to get it done at a tournament like this. I didn't feel like I could go out and try to compete and win a tennis match."
  
Nadal had already withdrawn from the 4.45 million dollar tournament while Federer and Juan Martin del Potro are playing with back and toe injuries.
  
Roddick is replaced by Radek Stepanek, who will face Federer later. The 27th-ranked Czech needs to beat both Federer and Simon to have any chance of reaching the semis.
  
Roddick also pulled out injured in 2005, when Nadal, Andre Agassi, Lleyton Hewitt and Marat Safin also missed the tournament, angering Chinese organisers.
  
The eight-man season finale is played in a round-robin format with the top two players from Red and Gold Group reaching the last four.