LONDON (Reuters) - Ruthless world No. 1 Novak Djokovic thrashed Stanislas Wawrinka 6-3 6-0 as he closed in on a stunning climax to his season at the ATP World Tour Finals on Wednesday.

The Serb responded to losing the first two games to steamroller the Swiss who was unfortunate to meet the defending champion at his imperious best. Djokovic tops Group A with two wins and is now poised to qualify for the semi-finals.

Djokovic, who will clinch the year-end number one ranking should he beat Berdych in his final round-robin match, dialled in his A game to counter an aggressive start by Australian Open champion Wawrinka, winning 20 of 23 points in a five-game burst from 0-2 down in the first set.

Wawrinka, a semi-finalist here last year, was run ragged at times by the power, precision and finesse of the 27-year-old Djokovic who racked up a 29th consecutive indoor victory with laser-guided forehand winner.

All eight round-robin matches have so far been decided in straight sets, a theme continued by the Czech Republic's Tomas Berdych, who rebounded from his opening match defeat to Wawrinka with a 6-3, 6-1 victory over disappointing U.S. Open champion Marin Cilic.

Cilic, making his debut appearance at the showpiece event this year having won the U.S. Open, never looked comfortable from the moment he dropped serve in the first game. The 26-year-old Croat did have two break points to get back to 4-4 in the first set but Berdych held form and went on to dominate the rest of the match. Cilinc has so far managed to collect only six games at the elite year-ender in the O2 Arena.

American world No. 1 doubles duo Mike and Bob Bryan also got back to winning ways as they came from a set down to beat Jean-Julien Roger of the Netherlands and Horia Tecau of Romania.