Both men have made it to the final before.
The tournament in Qatar has also been the scene of Monfils' only triumphs over Nadal, who lost to the Frenchman in the quarters in 2009 and in the semifinals in 2012.
Nadal, the only seeded player to reach the semis, lost the first set Friday to German dark horse Peter Gojowczyk, ranked 162nd in the world, but took charge in the second set to win 4-6, 6-2, 6-3.
Monfils, meanwhile, needed less than a hour to dispatch Germany's Florian Mayer 6-3, 6-2.
The Frenchman will be looking for only the fifth tournament victory of his career.
For the 27-year-old Nadal, a title in Doha would be his first-ever in the initial week of the season and boost his win total to 61.
"I never win a title in the first week of the season, and tomorrow I have another opportunity," the Spaniard said Friday. "A lot of times I have tried here so it will mean a lot to me to succeed finally."