Davydenko, Djokovic make semis in China

Action continued today at the Masters Cup in Shanghai. The gold group has now finished playing the round-robin portion of the tournament, and both Nikolay Davydenko and Novak Djokovic ended up with a 2-1 record, which means they both advance to the semifinals. In the red group, they'll wrap up round robin play tomorrow. Andy Murray is already 2-0, so he's locked up a spot in the semis, but Federer needs to win tomorrow against Murray in order to advance. Andy Roddick withdrew from the tournament with an ankle injury, and he was replaced by Radek Stepanek. I have no idea how or why they chose Stepanek, but it doesn't really seem fair that Murray had to play Roddick while Federer and Simon get to play Stepanek. It's a moot point since Murray won, but I wonder what would have happened if he had lost to Roddick. Anyway, Stepanek can't advance to the semis, so Federer controls his own destiny. I'm not quite sure why Federer would make the semis if both he and Simon win, because Simon beat him head-to-head. There must be some rule I'm overlooking. Round-robin tournaments are pretty rare, so I'm not up-to-date on how exactly they're run.