Roger Federer will miss Switzerland's Davis Cup series against the United States and next week's tournament in Dubai as he recovers from a back injury.Federer said on his Web site Tuesday he did not have enough time to completely strengthen his back after hurting it last fall and is taking the break as a precautionary measure."I will use the coming weeks for further rehabilitation in order to be sure that I'm fully fit for the rest of the season," Federer said.Switzerland faces the United States in the first round of the Davis Cup on March 6-8 in Birmingham, Alabama."This decision was very difficult for me, because I won't just be missing one of my favorite tournaments of the tour, but also the chance to go through to the next round with my teammates," Federer said.Federer, who is ranked No.