The sixth seed got off to the worst possible start when he was broken in the very first game of the opening set, but he soon restored parity with a break back in the fourth game.
Monfils immediately responded with another break to regain control of the set, however, and the Frenchman went on to serve out the remainder to take the opener 6-4.
It was the first time that Nishikori had dropped a set in the tournament, but he quickly responded and a break in the fourth game of the second saw him race into a 4-1 lead.
Monfils put up a fight by winning his remaining two service games, but Nishikori held his own and clinched the second set 6-3 at the first time of asking.
There was another early break for the Japanese player in the deciding set too as he moved into a 3-1 lead on this occasion, but when serving for the chance to go within one game of victory, he allowed Monfils back into the match.
The Frenchman broke to level things up at 4-4 before holding off a Nishikori fightback in the next game to find himself receiving for the set.
He looked destined to finish it after racing into a 40-0 lead too, but Nishikori fought back and survived four match points before restoring parity once again at 5-5.
Monfils won the next game with minimum fuss but failed to take a fifth match point in the final game as Nishikori once again held on to force a tie-break.
Monfils was left to rue those missed opportunities as he managed to win just two points on serve in the tie-break, with Nishikori wrapping the match up at the first time of asking.