The Scot saw off Nishikori in straight sets to set up his fifth semi-final appearance.
"I thought it was a good match, there were a lot of good points," Murray said. "Most of the fun points he was winning so I was trying to keep them as short as possible but I did well.
"He's had a few long matches and I thought the first two sets he fought really well, there were a lot of close games and I played better in the third. It was tough, I think he played two five-setters and a four-setter in the last few rounds."
Murray, ranked fourth in the world, identified his serve as an area to work on ahead of his next match.
"I think I need to serve better," he said. "I didn't serve particularly well but the returning was good."
Murray will face either Novak Djokovic or David Ferrer in the semi-finals.