Nishikori crushed Martin Klizan 6-1, 6-2 on Saturday to book his spot in the final for the second successive year.
"I’m delighted to be playing the final again," Nishikori told the official ATP site. "You don’t usually play two finals in a row, especially in such a difficult tournament, with many great players."
Nishikori also revealed that he started to become more aggressive and changed his style of play on clay after winning the Barcelona Open in 2014.
"Lifting the trophy here last year gave me a lot of confidence and I think I changed my way of playing on clay," he said. "I’ve become more aggressive, my service is better and so is my return.
"I guess my style is becoming more Spanish. I still feel more comfortable on hard court than on clay but these last four matches have been great and I think they’ve helped me improve."