The Serbian beat American Alex Michelsen in a thrilling two-set match that finished 7-6 (7/3), 7-6 (11/9).

Meanwhile Jannik Sinner and Carlos Alcaraz showed why they were top in the world as they raced through to the next stage with confident straight-set wins.

Sinner dispatched Japan's Taro Daniel 6-1, 6-4, while Alcaraz made short work of China's Shang Juncheng, winning 6-2, 6-2.

Four-time Shanghai champion Djokovic did not have it quite so easy against 43rd ranked Michelsen.

Introduced as the "Greatest Of All Time" as he walked onto the court, he received a rapturous welcome from the packed stadium.

But Michelsen started the first set strongly, breaking Djokovic early and going 1-4 up –- to the displeasure of the crowd, fully behind the 37-year-old former world number one.

To their delight, the Serbian levelled, and then hit his stride in the tiebreak, winning it 7-3.

The reverse happened in the second set when it was 20-year-old Michelsen's turn to catch up to take it to the tiebreak.

Djokovic admitted to being "surprised" by Michelsen.

"It took me a little bit of time to get the rust off and to start feeling better on the court," he said.

"I was very glad to keep calm when it mattered in both tiebreaks."

The crowd, already in a frenzy after Djokovic spoke Mandarin at the end of the match, went wild as he showed off a new trick -- a sentence in the Shanghainese vernacular.

Unruffled Sinner

World number one Sinner has said he is not in a "comfortable" situation thanks to a World Anti-Doping Agency (WADA) appeal a decision by tennis authorities to clear him of wrongdoing after he twice tested positive for a steroid in March.

His recent blistering winning streak was broken by Alcaraz in the China Open final on Wednesday.

But there was no tiredness on display from either player on Saturday, with 23-year-old Sinner looking completely unruffled in the first set against the 93rd-ranked Daniel.

Daniel fell behind again in the second set and despite rallying halfway through to gently test Sinner, the Italian kept his cool, ending the set 6-4.

"I had only one practice session yesterday but I felt very comfortable on the court," he said after the match.

Sinner will face Argentina's Tomas Martin Etcheverry on Sunday.

"It will be a tough one tomorrow, very physical, because me and Tomas know each other quite well now," he said.

Earlier, arch-rival Alcaraz also looked comfortable throughout, breaking in the first game, with teenager Shang unable to make much headway against the world number two.

Despite their enthusiasm for Shang, the crowd could not resist Alcaraz's charm.

At the Spaniard's encouragement, they broke into loud cheers when he won a particularly exciting rally in which he hit a tricky backwards shot to keep the point alive.

"I just had one practice then was straight into this match, so to be able to show this level in the first match, I'm just really proud," 21-year-old Alcaraz said after the match.

Alcaraz will next face another Chinese player, wildcard Wu Yibing, who beat 25th seed Nicolas Jarry in the second round.

Rublev loses

The tournament lost its second top-ten player on Saturday, as Andrey Rublev fell to 19-year-old Czech Jakub Mensik, joining Norwegian Casper Ruud on the notable casualties list.

The Russian, a finalist at last year's Shanghai Masters, roared in jubilation after winning a tiebreaker to take the first set.

But 65th-ranked Mensik broke Rublev in the first game of the second set, and again in the fifth to draw even.

The Czech then came from behind in the third to beat the world number six for the second time this year, with a final score of 6-7 (7/9), 6-4, 6-3.