Australia's final two men involved at the Shanghai Masters have bowed out in the round of 16.

After Jordan Thompson, Chris O'Connell, Rinky Hijikata and Li Tu were knocked out in rounds one and two, compatriots Alexei Popyrin and Aleksandar Vukic suffered the same fate on Tuesday.

Popyrin was beaten 7-6 (7-5), 6-2 by ninth seed Grigor Dimitrov in one hour 30 minutes, while Vukic went down 6-4, 6-2 to Czech Tomas Machac.

Neither Popyrin nor Dimitrov faced a break point in the opening set, which the Bulgarian took in a tiebreaker.

The 20th seed was broken twice early in the second and could not recover, and fellow Sydneysider Vukic lost his serve three times to go out after winning twice in qualifying and then seeing off eighth seed Casper Ruud in the previous round.

Stefanos Tsitsipas of Greece had to wait out a two-day rain delay to beat Alexandre Muller 6-3, 7-5 and set up a showdown with world number five Daniil Medvedev.

Novak Djokovic produced a clinical display to beat Flavio Cobolli. The four-time champion needed only 64 minutes to dispatch the Italian, who had played a two-and-a-half match with Stan Wawrinka on Monday (6-1, 6-2).

Djokovic, playing his first ATP Tour event since the US Open, had looked a little rusty in his first match against Alex Michelsen but was sharp from the start this time.

"From the beginning I had a clear game plan of what I need to do, staying aggressive and use every short ball to take the initiative in the point, and I've done that," Djokovic said, adding he thought Cobolli "was evidently exhausted".

Djokovic now meets Roman Safiullin who upset 13th seed Frances Tiafoe 5-7, 7-5, 7-6 (7-5).

The American launched an expletive-laden rant at the umpire following his defeat, having been given a time violation at 5-5 in the deciding tie-break.

Alexander Zverev also used expletives in an exchange with the umpire. A double bounce of the ball was correctly called when Zverev stretched to play a shot approaching the net.

Zverev converted his eighth match point to beat Tallon Griekspoor 7-6 (8-6), 2-6, 7-6 (7-5).

Elsewhere, Gael Monfils defeated French compatriot Ugo Humbert 7-6 (9-7), 2-6, 6-1 as the 38-year-old became the second-oldest player ever to reach the last 16 in Shanghai. He next meets third seed Carlos Alcaraz.

Ben Shelton sealed a straight-sets win over Roberto Carballes Baena and faces top seed Jannik Sinner next while Taylor Fritz got the better of Japanese qualifier Yosuke Watanuki 6-3, 6-4.

AAP/AP/DPA