The fourth seed needed just 63 minutes to beat Troicki, with the Serb having no answers to Nishikori's polished all-round game.
The 25-year-old Nishikori sent down five aces and hit 16 winners against an opponent who was struggling with a side injury which required treatment.
Nishikori broke Troicki three times in the opening set, although he would surrender one break, before adding a further two breaks to his tally in the second set.
The Japan ace, who made the semi-finals of the tournament last year, will face 18th seed David Goffin in the round of 16.
Fifth seed Milos Raonic had a much more challenging time of it, the Canadian needing three sets to see off Jeremy Chardy of France 6-1, 5-7, 7-6 (3).
After easing through the first set, Raonic served for the match at 5-4 in the second, but Chardy was in no mood to go down without a fight and rattled off consecutive breaks to take the set.
With neither player able to grab a break in the decider, although Raonic staved off two break points, the set headed into a tiebreaker, and it was here that Raonic just found the edge, taking the breaker 7-3 and with it the match.