US Open champion Juan Martin del Potro kept alive his challenge at the ATP World Tour Finals with a 6-4, 3-6, 7-6 (7/1) victory over Fernando Verdasco Tuesday. After losing his opening Group A match to Andy Murray, Del Potro knew he had to see off Verdasco in their first ever Tour meeting to remain in contention for a semifinal place. The 21-year-old rose to the challenge with a typically belligerent display of guts and power-hitting and could qualify for the last four with a win in his last match, against Roger Federer, Thursday. Verdasco, 26, is appearing in the Tour Finals for the first time after enjoying the best season of his career. But the left hander looks certain to make an early exit after his second defeat of the round robin stage. Del Potro said: “I served much better than my other match. I was very focused in the important moments, especially in the tiebreak and I took my opportunities.” Verdasco added: “I played badly in the tiebreak and he didn't make mistakes so that's why I lost. He didn't play even one ball long and I played maybe four trying to move him around. “It was a big opportunity missed. Right now the chances are pretty low for me to qualify. Many things need to happen for me to qualify.” Suffering a nose bleed in the first set against Murray had hardly helped del Potro's cause Sunday, but he packed a greater punch in the early stages this time. The towering Argentine has risen to fifth in the world rankings after a breakthrough year capped by his shock five-set victory over Federer in September's US Open final and he showed the form that so impressed in New York as he took control of the first set. Defending champion Novak Djokovic handed Nikolay Davydenko another painful defeat at the ATP World Tour Finals as the Serb secured a 3-6, 6-4, 7-5 victory Monday. Djokovic beat Davydenko to win the final of this prestigious end-of-year event 12 months ago and the world No. 3, who also defeated the Russian in the group stage last year, came from behind to frustrate him again in his opening Group B match at London's O2 Arena. Djokovic said: “He was a better player in the first two sets and maybe overall in the whole match. But when you have bad days, you still find a way to win. I think that's something that is great to have in your game.” After reaching the final last year, Davydenko is in danger of being eliminated at the group stages this time and will have to beat Nadal Wednesday to keep his hopes alive. Davydenko's own chances of victory could be hampered by breathing difficulties that have bugged him for several weeks. He called for a doctor in the second set because of the problem and added: I have a problem with my breathing. I already see doctors in three different countries. It's really a problem. “Some doctors say maybe asthma, some say maybe allergy, some say bronchitis. I really don't know what it is.”
