Kei Nishikori has set his sights on usurping Novak Djokovic at the top of the ATP Tour world rankings.

Nishikori currently sits at number five, after rising to a career-high four earlier this month, after an early exit at Indian Wells.

Djokovic, Roger Federer, Rafael Nadal and Andy Murray have dominated at the summit of men's tennis for the last decade or so, but Nishikori is determined to rise higher.

"It's going to take some time to get number one," said the Japanese.

"Novak is the top player right now. It's not easy to beat him or Roger or Andy. 

"But, slowly, I think I'm coming. I'm playing good tennis and feeling much stronger mentally and physically, too. 

"It might take some time, but hopefully I can get there [number one] as soon as possible."