The Japanese world number 12 beat his Colombian opponent 6-2 6-2 on Sunday to clinch the first clay title of his ATP career.
"I was playing well," Nishikori told reporters. "I felt I should be in the final and I'm very happy to win.
"I was good on clay when I was little. I was winning all the European juniors events. Now it's much tougher than hard courts and I was struggling a bit, but I've been playing well these past couple years so there's no fear to play on clay."
The title is Nishikori's second of the year after his victory at the National Indoor Championships in February.