BRISBANE (AFP) - Japan's Kei Nishikori overcame a slow start to cruise past Mikhail Kukushkin and move into the quarter-finals of the Brisbane International on Wednesday.

Nishikori lost his opening serve to go 2-0 down in the first set but soon recovered to get on top of Kukushkin and ease to a 6-3, 6-4 win.

"I thought it was a great match," the second seed said. "(But) I didn't start quite well - I think he really started well."

Nishikori, who reached the final of the US Open in 2014, was below his best in his first match of the season, particularly during the opening set. His serve was initially poor but it improved as the match progressed and he faced only one break point in the second set.

It was Nishikori's fourth straight win over Kukushkin and ensured he has reached at least the quarter-final stage in Brisbane for the fourth time in five years.

The world No. 8 will now play either seventh-seeded Australian Bernard Tomic or Czech veteran Radek Stepanek in the last eight.

Big-serving Croatian Marin Cilic, who beat Nishikori to win the 2014 US Open, downed rising Korean star Chung Hyeon in two tight sets 7-5, 7-6 (7-3).

Cilic will take on Austrian Dominic Thiem after the eighth seed's comfortable 6-3, 6-2 win over Denis Kudla of the US.