World Number One Novak Djokovic and Stanislas Wawrinka have booked the semi-final berths of the ATP World Tour Finals in London. In their final Group-A matches Friday night, while Djokovic defeated Tomas Berdych in straight sets, 6-2, 6-2, Wawrinka edged past Marin Cilic, 6-3, 4-6, 6-3.Earlier from Group-B, World Number Two Roger Federer and Kei Nishikori had advanced to the last-4 stage.For a place in the final, Wawrinka will take on Federer, while Djokovic will play Nishikori.