It's been a disappointing day for Australia's final two men involved at the Shanghai Masters.

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 in the round of 16 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 couldn't 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 No.5 Daniil Medvedev.

It is the fourth time this year that 12th-ranked Tsitsipas has made this stage of an ATP tournament, but it will be his first meeting in almost a year with Medvedev.

Tsitsipas and the former US Open champion will meet for a 14th time, with the Russian enjoying a commanding 9-4 head-to-head lead.

"I consider him someone that I respect on the tour, much more than I did before," Tsitsipas said.

"We've had some heated things on the court in the past, but I think those things have resolved themselves over time, and we also had the time to speak about those things and have a common understanding of why these things happen."

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.

"It's never easy to play Ugo, he's really aggressive, playing fast off both wings and takes a lot of time from you," said Monfils, who next meets third seed Carlos Alcaraz.

Ben Shelton sealed a straight-sets win over Spain's Roberto Carballes Baena and faces top seed Jannik Sinner next up in a repeat of last year's last-16 clash surprisingly won by the American.

Seventh seed Taylor Fritz got the better of Japanese qualifier Yosuke Watanuki 6-3 6-4.

Novak Djokovic faces Flavio Cobolli later as German Alexander Zverev plays Tallon Griekspoor.

With AP & DPA.