The 35-year-old German won his second trophy of the season after triumphing in Munich in May.
"This victory is really something special," the world number 12, who also won in Vienna 12 years ago, told reporters. "You don't win tournaments too often and this year I've won in my two favourite cities."
Haas, whose career seemed all but over after he suffered a series of shoulder, elbow and hip injuries, is certainly relishing all the unexpected success he has enjoyed in recent months.
"If you'd seen me 3-1/2 years ago ... I was really asking myself if it made sense to continue," Haas said. "I have been paying a lot of attention to my fitness and nutrition and I am always looking to improve my tennis."
Haas's never-say-die attitude certainly helped him out of a hole when he trailed Dutchman Haase 4-2 in the final set.
A Haase double fault gave him the break back in the next game and from then on there was no stopping the German, who completed the win when Haase flicked a backhand long.
Haas, currently 12th in the ATP Race, only has an outside chance of making it to next month's eight-man season finale at London's O2 Arena.
Rafa Nadal, Novak Djokovic, David Ferrer and Juan Martin Del Potro have qualified for the tournament and Wimbledon champion Andy Murray has pulled out following back surgery.
Tomas Berdych, Stanislas Wawrinka, Roger Federer, Richard Gasquet, Jo-Wilfried Tsonga, Milos Raonic and longshot Haas are in a battle to secure the remaining four spots.
- Sports & Recreation