After a great first set Monfils game dropped drastically and the third seed Djokovic pounced and wrapped up the match for the loss of just three more games 7-6 6-1 6-2.
"The first set was one of the turning points," said Djokovic.
"He had a break advantage and I managed to come back. I was focused when I needed to be. These are the worst conditions so far. I'm fortunate to be in the semi-finals."
Djokovic, now with a 5-0 head-to-head against Monfils is in the semi-finals of the US Open for the fourth consecutive year and will now play Swiss Roger Federer who saw off the challenge of Robin Soderling 6-4 6-4 7-5.
