Novak Djokovic kept his hopes of winning a 100th career ATP title alive with a straightforward 6-3 6-3 win against Gael Monfils in the Brisbane International last 16.

The Serb, 37, will face Reilly Opelka in the last eight after the American's 7-6 (11-9) 7-6 (7-4) victory against Italy's Matteo Arnaldi.

Djokovic took his only break point opportunity in the first set, helping him to race into a 4-1 lead and to take the set within 36 minutes.

Monfils, 38, missed a simple forehand on serve at 1-1 and the Frenchman failed to take a point in the remainder of the game as Djokovic broke early in the second set.

Djokovic held serve three times from there and broke Monfils again at 5-3 to wrap up his 20th career win against the Frenchman.

Opelka awaits Djokovic in the quarter-finals, when the pair will play each other for the first time.

The 24-time Grand Slam winner says he will have to tailor his game to face the big-serving American, who hit 25 aces against Arnaldi.

"Opelka has been injured and he struggled a lot for a couple of years," said Djokovic.

"My gosh, I'm going to have to probably step back a few steps for that first serve."