Novak Djokovic comfortably cleared the first hurdle to retaining his world number one ranking with victory over Gilles Muller in Paris.

The Serbian needs to reach the final of the BNP Paribas Masters, a tournament he has won for the last three years, to ensure he will hold off Andy Murray at least until the ATP World Tour Finals later this month.

Djokovic has struggled for form and motivation since completing his career Grand Slam at the French Open in June, when his lead in the rankings appeared unassailable.

He arrived in Paris after a two-week break without either of his coaches Boris Becker or Marian Vajda but declaring himself rejuvenated.

After an impressive first set against Luxembourg's Muller, Djokovic was less impressive in the second but saved a break point in the fourth game and went on to win 6-3 6-4.

He is likely to face a stiffer test next against Grigor Dimitrov.

The race to qualify for the World Tour Finals in London was opened up by a 6-2 6-4 defeat for sixth seed Dominic Thiem against Jack Sock.

(Getty (Getty)

Thiem is currently seventh in the standings - with the top eight men to qualify - but could yet be overtaken.

Marin Cilic, one place behind the Austrian, continued his winning run after victory in Basle last weekend by beating his fellow Croatian Ivo Karlovic 7-6 (9/7) 6-2.

He next plays another of the men in contention, David Goffin, who was a 7-6 (7/5) 6-3 winner over France's Nicolas Mahut, while Jo-Wilfried Tsonga defeated Albert Ramos-Vinolas 6-3 6-4.