"Physically I feel really good, and I'm moving and feeling better match after match," said Wawrinka.
"Tonight was a tough match. I had to really fight at the end. I was a bit nervous at the end of the match. We had some long rallies. It's good to see that I can keep being there, mentally, physically."
Nishikori took a step closer to claiming his second title of the year by defeating Hungary's Marton Fucsovics 6-3 6-2.
Russian fifth seed Daniil Medvedev dismantled Frenchman Jo-Wilfried Tsonga 6-4 6-2.
He will face another Frenchman in Gael Monfils, who beat Bosnia's Damir Dzumhur 6-1 6-2.
(Reporting by Julien Pretot, editing by Pritha Sarkar)
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