Novak Djokovic was relieved to edge past Alex Michelsen at the Shanghai Masters as his quest for a 100th career title got up and running. 

Djokovic, who has won the tournament in China on four occasions, needed almost two hours to overcome his American opponent on Saturday, eventually doing so 7-6 (7-3) 7-6 (11-9).

The Serbian's slow start was punished by Michelsen, who raced into a 4-1 lead in the opening set, with Djokovic managing just three points from the first three games. 

But the 24-time grand slam champion rallied to take the first set to win his 11th tie-break from the 15 he has played this season.

"It was the first match against Alex and I also hadn't played in a while, so it took me a little time to get the rust off," said Djokovic after his first match sine the US Open.

“He started off terrifically; big serves and an aggressive style of tennis, he's not afraid to step it up and take it to his opponent."

Djokovic then led 4-1 in the second, only to allow Michelsen back into the contest, but the Serbian held his nerve in the decisive tie-break, saving two set points to advance. 

The 37-year-old has now won 35 of the 40 matches he has contested at the ATP Masters 1000 event in Shanghai - more than any other player.

Djokovic is also the only player in history (since 1990) to win all nine ATP Masters 1000 titles, a feat he has achieved twice on his way to a record 40 titles at this level.

He is yet to reach a final in the series this year, with the Serbian acknowledging the difficulties he faced against his American opponent. 

"I was 4-1 up, 15/40, and he hit the net-cord that went over. It could easily have been 6-1 for me in the second,” Djokovic said.

"But at this level, things can change very quickly.

"It was a very close encounter and I thought a high level of tennis in both sets. I'm just glad to keep calm when it mattered in the second-set tie-break.

"I'm glad to really be challenged by a young player, and I'm looking forward to the next one."

Up next for Djokovic is either Italian Flavio Cobolli or wildcard Stan Wawrinka.