The Japanese player was extended to a third set by Janowicz, but produced a determined response to seal a??6-4 5-7 6-3 win in Halle.
The precise Nishikori appeared to have the edge over the powerful Pole when he seized the opening set, but Janowicz staged a gritty fightback in the second set to level the match.
Faced with a decider, Nishikori simply resumed his assault on the Janowicz serve, and calmly closed out the victory after forging ahead.??
He awaits the winner of the last quarter-final between French fourth seed Gael Monfils and Italian Andreas Seppi.