Federer seems to have the upper-hand against Wawrinka on hard courts right now. Neither the US Open semi nor the WTF semi was particularly close. But I find it hard to see Federer beating Wawrinka again on clay if Wawrinka can bring his A-game. On clay, Stan's weight of shot off the ground is just going to be prohibitively difficult for Federer to do anything against. (I think Federer should consider skipping the clay-court season next year. I think he's finished as a threat on clay. And at his age, he'll need to be fresh if he's to challenge at the summer events. If he's still on form, he might have a chance at winning one of Wimbledon, the Olympics, and the US Open. But he will need to plan his schedule carefully).