London, Nov 21 : Dominic Thiem outlasted world no.1 Novak Djokovic in a gripping last-four clash to reach his second successive final at the ATP Finals in the O2 Arena in London. Thiem, ranked third behind Djokovic and Rafael Nadal, won the match 7-5, 6-7(10), 7-6(5). Thiem broke serve at 5-5 in the first set and sealed it with an ace. Djokovic, however, grinded his way to victory in the second set, denying Thiem the win despite the Austrian getting four chances to seal the game. The third game was also pushed to a tiebreaker, which was started by Thiem but he went on to lose the next three points. He however roared back to win the next six points and while Djokovic saved a fifth match point, Thiem got the job done at the next opportunity. He will now face either Daniil Medvedev or Nadal in the final which is scheduled to be played on Sunday. /IANS