Rublev needed four match points to wrap up his 6-1, 3-6, 6-3 win over Nikoloz Basilashvili in a semifinal where both players landed fewer than half of their first serves.
That put the fourth-seeded Russian into his third final of the year after he won in Rotterdam in March and lost the Monte Carlo Masters to Stefanos Tsitsipas a month later.
"I can play on every surface, and I will try my best again tomorrow," said Rublev, who is 8-3 in career finals.
Humbert made it to his third career final by beating Felix Auger-Aliassime 6-4, 3-6, 7-6 (5). Canada's Auger-Aliassime saved two match points with Humbert leading 4-5 in the decider and two more in the tiebreaker before his French opponent sealed the win.
Humbert won both of his previous tour finals last year in Auckland and Antwerp, both on hard courts.
Auger-Aliassime was denied back-to-back grass finals after he lost to Marin Cilic in Stuttgart last week.