The 29-year-old Hewitt took a 4-1 lead after breaking Monfils' serve twice. He was serving for the set at 5-3 but Monfils fought back, forcing Hewitt to four set points in the 12th game before sealing it.
After trading breaks in the second set, Hewitt won the last four games to finish the match.
"It's as good as I could have done," Hewitt said. "The last two weeks, I've done everything possible for me."
Monfils needed treatment after struggling with what appeared to be a back or abdominal problem late in the second set.
"I felt something but I don't know (what it is) yet," Monfils said. "I hope it's not major. ... That's why I took it easy in the end. I started to feel it more, but it should be OK."
Austria's Jurgen Melzer beat Davydenko 7-6 (4), 6-2 in a playoff for third.