It's the third time that Monfils has pulled out of the ASB Classic before the event, and he appeared visibly upset during the press conference.
"I have a problem with my quadriceps," said Monfils. "I have a pain since two days so unfortunately I am not able to play or train. It's very tough in many ways.
"I'm disappointed with the situation I put myself through and I put also the tournament. Obviously this year is a bit tough because we had a couple of retirements. I love the city, the fans are always great. It was a tough decision for me [this afternoon] to say that I am not playing."
Monfils arrived in Auckland on Saturday and had completed two practice sessions, but felt pain in a subsequent session on Monday morning.
He is famously injury prone, and said he had never completed a full season.
"That's the paradox," said Monfils. "It's only three times here but I think I have never played a full season. It's been 16 seasons. It happens to me constantly. I'm sad definitely, for the tournament, for my fans but I am sad for me also. I have put in seven weeks of great work, I came earlier to Australia to be in shape and to come here and not be able to perform…it's tough.
Tournament director Karl Budge admitted the news was "devastating", though he said he would try to recruit the Frenchman again, despite his chequered record.
"I will," said Budge. "You want the guys that put on the most entertaining performances and he is that guy. I've give most things for him to walk out on centre court and play a match"
Pablo Cuevas, who was the sixth seed at the ASB Classic last year, will take the place of Monfils in the draw as a 'lucky loser'.
* Tonight, Canadian seventh seed Denis Shapovalov was upset 6-4 4-6 6-4 by 2017 finalist Joao Sousa, in an entertaining match that stretched on for almost two and a half hours.
