After nearly a year of global travel and dozens of matches, most of Novak Djokovic's rivals are complaining about their mental and physical fatigue. Unbeaten since losing the U.S. Open final to Rafael Nadal, Djokovic joined Nadal in the ATP World Tour Finals semifinals, overcoming Juan Martin del Potro on Thursday. After extending his winning streak to 19 matches with a 6-3, 3-6, 6-3 victory over the hard-hitting Argentine, Djokovic said he was playing the best tennis of his season. But, you know, I try to find that as an opportunity for me, you know, to get extra motivated to kind of push myself to the limit and deliver the goods when needed.'' Against del Potro, he delivered. More...