The Spaniard was made to work in a competitive opening set but breezed through the second to clinch a 6-3 6-2 triumph over his unseeded opponent.

His latest success on clay in the principality sees him lift the trophy for a third straight year and an 11th time overall, while his career tally of 31 Masters titles moves him one clear of Novak Djokovic in the record books. 

Nishikori, making his way back from a serious wrist injury, put up a fight in the early stages of Monday's (AEST) showpiece, eventually prevailing in a truncated opening service game before surprisingly registering the first break of serve to go 2-1 ahead.

However, Nadal responded to the setback in style. He broke back immediately to draw level on his way to winning four games on the spin.

Nishikori stopped the rot to make it 5-3 but was powerless to prevent his rival serving out for the set.

Nadal failed to capitalise on a break-point opportunity at the start of the second, though he did not have to wait long for another opening.

The 31-year-old held his own serve before reeling off five games in a row and while Nishikori made him wait by holding to love to make it 5-2, the No.1 seed would not be denied.

He served out to win after one hour and 34 minutes on court, in the process making it a career-best 36 successive sets won on his favourite surface.

By defending in Monte Carlo, Nadal made sure he will remain world No.1 for a 171st week, denying Roger Federer the chance to take over at the summit.