The 24th-ranked Japanese star, who is trying to make his way back into the top 10 after a wrist injury derailed the latter part of his 2017 season, continued his strong run of form in the clay-court campaign on Wednesday, reports Efe.
Nishikori's potent return game was the difference in the match, as he broke Dimitrov's serve five times in the contest and twice in both the second and third sets to take the match in two hours and 56 minutes.
Dimitrov won only 30 percent of his second-serve points in the second set, while in the decider the Bulgarian got in trouble due to a low first-serve percentage (48 percent).
Next up for Nishikori in Thursday third-round action will be Germany's Philipp Kohlschreiber, who defeated American Jack Sock 6-4, 6-3 on Tuesday.
Last month, the Japanese player reached the final of the Monte-Carlo Masters before losing to then-world No. 1 Rafael Nadal.
Like Monte-Carlo, the Italian Open is one of the biggest clay-court tournaments on the ATP World Tour and a key tune-up event for the French Open, which gets under way on May 27.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)