A disappointing year ended with another loss for Roger Federer. The 32-year-old Federer lost in straight sets to rival Rafael Nadal in the semifinals of the ATP World Tour Finals on Sunday, ending a season in which he has slipped to No. 7 in the rankings and lost in the second round of Wimbledon and the fourth round at the U.S. Open. ”I think something is possible for next year,” Federer said after his 7-5, 6-3 defeat to Nadal, his first loss to the Spaniard on an indoor hard court. ”It’s important to stay within a certain ranking,” Federer said.