MELBOURNE (REUTERS) - Former US Open champion Marin Cilic said his 7-5, 7-6 (7-3), 3-6, 6-3 win over Russian fifth seed Andrey Rublev at the Australian Open on Saturday (Jan 22) was the result of a gradual improvement in form over the past few months.

Cilic, who reached the semi-finals of both tune-up events in Adelaide, has continued his strong form from the end of the previous season when he won the St Petersburg Open and reached the finals of the Kremlin Cup and Davis Cup with Croatia.

"Just generally, as a player, I'm that kind of player where I need to build my form gradually," the 27th seed told reporters.

"Started somewhere in the summer last year, had some good matches, good wins, and played some quite good tournaments end of the season. Also in the start of this season were two good tournaments in Adelaide.

"That's extremely important... because doesn't matter in the end who I play, if I'm playing the best I can, it's a great opportunity for me to beat almost everybody on the Tour."

The Croatian's second win over Rublev in six meetings saw him reach the fourth round at Melbourne Park for the eighth time, where he will face Canada's ninth seed Felix Auger-Aliassime.

"I'm expecting absolutely another difficult challenge ahead. Felix started the season very well," Cilic said. "They (Canada) won the ATP Cup and today I saw he played incredibly well against (Britain's Dan) Evans.

"We know each other. We played a few times, and he's (a) big hitter, big serve, big forehand."

Auger-Aliassime breezed past 24th seed Evans 6-4, 6-1, 6-1.