One reason Federer struggled against Seppi was due to a back injury, which Federer said was a muscle spasm. Although he did play against Seppi, he wasn’t able to hit as hard as he was accustomed which may explain how Seppi took a set off the Swiss maestro. Federer said his back hadn’t improved enough to play his match against Jo-Wilfried Tsonga.
This leaves Nadal to play Gael Monfils in the other semifinals with Tsonga the automatic finalist.