Australian tennis star Christopher O'Connell has continued his recent impressive form, advancing to the round of 16 at the Japan Open.

O'Connell, who made the third round at the recent US Open before losing to eventual champion and world No.1 Jannik Sinner, scored an impressive 6-4 7-6 (7-5) victory over Argentina's Tomas Martin Etcheverry in their round-of-32 clash in Tokyo on Thursday.

The Aussie 30-year-old has risen to a personal-best ranking of No.53 in the world on the back of consistent results this year.

On Thursday O'Connell proved too good for his Argentine opponent, ranked 18 places higher at No.35.

The Australian sent down eight aces to Etcheverry's two, riding his powerful serve and groundstrokes into a round-of-16 clash with American Alex Michelsen, who upset world No.12 Stefanos Tsitsipas, of Greece.

O'Connell's victory came on a mixed day for Australia, with compatriot Alexei Popyrin losing in three sets against Czech player Tomas Machac.

There had been high hopes for Popyrin, who won his first ATP Masters 1000 event in Canada in August, then stunned former world No.1 Novak Djokovic on his way to the round of 16 at this month's US Open.

Popyrin's impressive results this year have seen his ranking rise to a career high of No.23.

However, Machac defied his own ranking of No.34 to beat Popyrin 6-3 6-7 (5-7) 7-5.

Another in-form Australian, Jordan Thompson, winner of the men's doubles at the US Open, was to play world No.9 Casper Ruud, of Norway, later on Thursday in this ATP Tour 500 event.

In other results on Thursday, top-seeded American Taylor Fritz was upset by No.24-ranked Frenchman Arthur Fils 6-4 3-6 6-3.

Fils peeled off 32 winners in a power-hitting display against Fritz, who made his first grand slam singles final at this month's US Open this month.

The Frenchman next faces former top-10 player Matteo Berrettini, of Italy.

Defending champion Ben Shelton shook off a rusty first set to beat fellow American Reilly Opelka 3-6 6-1 6-4.

Brandon Nakashima won another all-American tie against US Open semi-finalist Frances Tiafoe 7-5 6-3.

Other winners included fifth-seeded American Tommy Paul and US Open semi-finalist Jack Draper, of Britain.

- with AP