The Japan ace recorded a straightforward 7-5, 6-4, 6-4 win over Thomaz Bellucci on Wednesday despite not being at his best during the encounter.
However, while there is room for improvement, Nishikori is satisfied with where his game is at present.
"I'm feeling really good [about my first] two matches and winning three straight sets against a tough player like Thomaz. I'm very happy to be going to the next round," he told the official ATP website.
"This year I'm feeling comfortable and very confident, and my body is good. [My] mentality is also good, so I'm very focused. I hope to try to do well here, and let's see what happens."
Next on the agenda for Nishikori is a clash with Benjamin Becker after the German downed 32nd seed Fernando Verdasco 6-4, 0-6, 1-6, 7-5, 10-8.