LONDON (AP) -- Novak Djokovic lost the first two games of his match against Stan Wawrinka. After that, there was no stopping the world No. 1.

The top-ranked Serb stayed calm, withstood the early assault from the third-seeded Swiss, and then crushed him 6-3, 6-0 at the ATP Finals on Wednesday to close in on the coveted year-end No. 1 spot.

''I thought he played very well the first two games. But I wasn't frustrated. After that, it was a really amazing performance,'' Djokovic said after extending his unbeaten run indoor to 29 matches.

The two-time defending champion is in blistering form this autumn, and his game is perfectly suited to the O2 Arena's center court, where he has dropped just five games in two matches.

''I'm not worried,'' Wawrinka said. ''All the players who faced him recently have lost in two sets.''

Djokovic will be guaranteed to finish top of the rankings for the third time in four years if he wins his final round-robin match at the elite tournament against Tomas Berdych. He has two wins in Group, A while Berdych posted his first victory against debutant Marin Cilic. Before the last round of matches, all four players can still make it to the last four.

Wawrinka, who had dominated the Serb en route to his Australian Open triumph earlier this year, opened confidently, putting Djokovic on the defensive by dictating from the baseline.

Helped by some superb backhands, he converted his third break point in a tight first game that lasted seven minutes, then held for a 2-0 lead after hitting an excellent crosscourt pass.

But Wawrinka's momentum didn't last long. While Djokovic stayed focused, his opponent's game suddenly went missing as the Serb won the next five games - a run that included 11 consecutive points.

Wawrinka saved a set point in the eighth game with a good passing shot. But Djokovic never looked back after that, losing only seven points in the second set as Wawrinka was overwhelmed by the sheer pace of the two-time defending champion.

Berdych, who lost his opening match to Wawrinka, recovered to stay in contention for a spot in the semifinals by beating Cilic 6-3, 6-1.

Following his straight-sets loss, the Czech looked better against Cilic, hitting with accuracy and serving well on the big points.

''I think today was more fighting and getting through. It's good to have a win and it always counts,'' said Berdych, who is competing at the season finale for the fifth time.

Berdych has never won his opening match of the round-robin stage, and never lost his second match.

''It's not my first year. I have the experience of losing the first match in the past and I know how to come back. I think that was the biggest difference today,'' he said.

Berdych broke twice in the first set as the U.S. Open champion - who is now 0-2 - struggled with his serve.

Cilic, who has beaten Berdych twice this year at Wimbledon and the U.S. Open, got off to the worst possible start as he dropped serve immediately. The hard-hitting Croatian produced some good winners but was inconsistent throughout, failing to convert any of his six break points.

In the second set, Cilic held to level at 1-1 but hardly fought as Berdych won five consecutive games to wrap up the match. Cilic, one of three debutants at the O2 Arena, was routed 6-1, 6-1 by Djokovic in his first match and has won only six games in two matches.

''Let's put it this way, with the way he played, he would never win the U.S. Open,'' Berdych said.