Nishikori, Asia's top-ranked player, advanced when 2009 champion Nikolay Davydenko retired after losing the first set of their quarterfinal 6-4. Davydenko also pulled out of the quarterfinals here last year with an injury.
"It's sad to see him retire, but hopefully he is fine," Nishikori said.
Julien Benneteau of France also advanced, defeating Colombia's Alejandro Falla 6-2, 6-2. He will face either top-seeded Spaniard David Ferrer or Igor Sijsling of the Netherlands.