The Serbian came out on top in an absorbing clash on Saturday (AEST), stretching his winning streak over Nishikori to 13 matches to reach the last four of a tournament for the first time in 2018.

The 2-6 6-1 6-3 result books a 51st career meeting with top seed Nadal, who was also taken the distance in his quarter-final clash with home favourite Fabio Fognini in Rome.

Djokovic leads the head-to-head 26-24, though clay-court king Nadal has triumphed in 15 of their 22 previous duels on his favoured surface.

Still, the performance against Nishikori offered further encouragement for the former world number one after a difficult season so far that has seen him struggle for both form and fitness.

He endured a slow start to the delayed quarter-final in the Italian capital too, losing his opening service game in a one-sided first set that lasted 37 minutes.

However, Djokovic responded to the setback by racing out to a 4-0 lead in the second. Even though he was broken to end the streak, he hit back immediately with a break of his own before wrapping up the set with a sensational forehand winner.

Nishikori gifted his rival an early advantage in the decider when slamming an overhead smash straight into the net, yet recovered to get back on serve as he moved 3-2 in front.

However, Djokovic was not to be denied in what was a frenetic encounter, producing some sublime tennis when it mattered to take the final four games on the spin.