World Number one Novak Djokovic of Serbia is bracing himself for what he says ultimate challenge in Tennis when he tackles Spain's Rafael Nadal for the 37th time in today's U.S. Open which is the fourth and final Grand Slam of the season. The match will be Djokovic's third major final of the season after seeing off Andy Murray, who was the defending champion in New York, before being overwhelmed by the Briton at Wimbledon. Djokovic is already guaranteed to retain the world number one spot whatever happens today. Nadal on the other hand will attempt to crown the year's most compelling comeback with a victory that once would have seemed unlikely.Earlier, World Number One American Serena Williams retained the Women's Singles title, beating Victoria Azarenka of Belarus, 7-5, 6-7, 6-1. India's Leander Paes and his Czech teammate Radek Stepanek clinched the Men's Doubles title of the US Open beating second seeded Austrian-Brazilian pair of Alexander Peya and Bruno Soares, in straight sets, 6-1, 6-3 late last night.