The world number five triumphed in a war of attrition, eventually beating the former Wimbledon semi-finalists 6-4, 5-7, 6-3 to set up a match in Saturday's semi-final against Andreas Seppi.
"It was a really tough match," Nishikori said after the match.
"I am so happy to have won."
While Janowicz's big weapon is his serve, Nishikori had to call on all his creativity to match the big-hitting Pole, and eventually clinched the first set after managing a break of serve in the 10th game.
Janowicz responded with a break of serve early in the second set, and even though Nishikori levelled matters again shortly afterwards, the 2.03 m giant inched his nose in front again to force the match into a decider.
Nishikori, though, proved a more resilient player in the third set and set up his first match point with his eighth ace of the match. The following point Janowicz was unable to return another bullet serve to hand the 25-year-old the match.
"In the second set, my opponent was able to raise his game and my concentration level sagged," Nishikori added.
"At the end, I was able to find my strength again."