The 30-year-old Serbian had lost as many matches as he had won during a troubled 2018 so far but rediscovered his form with a 7-5, 6-4 triumph over the Japanese player at the Caja Mágica.

The triumph for Djokovic, who was forced to undergo a "small medical intervention" on his long-suffering elbow earlier this year, came at a valuable time with the French Open on the horizon.

Nishikori arrived on the red clay in the Spanish capital having reached the final of the Monte Carlo Masters, where he was beaten by world number one Rafael Nadal.

But the Japanese was unable to build on that performance as he crashed out of the event at the first hurdle following an error-strewn performance.

Djokovic will face either Britain’s Kyle Edmund or Daniil Medvedev of Russia, who clash tomorrow, in the second round.

Elsewhere in the men's singles event, 15th seed Lucas Pouille of France was beaten 6-2, 6-3 by compatriot Benoit Paire.

Milos Raonic of Canada, a finalist at Wimbledon last year, is also through after he dispatched Argentina's Nicolas Kicker 6-3, 6-2.

In the women's singles tournament, Denmark's Caroline Wozniacki, the world number two, survived a scare as she fought back from a set down to oust Australian Ashleigh Barty.

Wozniacki dropped the second set but recovered to clinch a 6-2, 4-6, 6-4 win to seal a place in the last 16.

Former world number one Maria Sharapova was among the other winners as the Russian beat Romania's Irina-Camelia Begu 7-5, 6-1.

The event continues tomorrow.