'This one feels so sweet,' said Tewell, who earned $240,000 for the win. 'It's just nice to get back in that winner's circle.'
Fleisher, who owns a stellar record when leading after the second round, also birdied the last to end at 13-under-par 203. He posted a final-round 68.
Tewell got off to a hot start when he birdied the first. He came right back with a birdie at the second as he holed out from a greenside bunker. Tewell also birdied the third from 10 feet out at Rock Barn Golf Club and Spa to quickly jump to minus-9.
The 54-year-old converted his fourth birdie at the par-5 seventh. After dropping in an 11-foot birdie try at the 10th, Tewell sank a 7-foot birdie putt at the 11th to take the lead from Fleisher, who bogeyed the same hole.
Tewell ran off four straight pars, while Fleisher joined him in the lead at minus-12 with a 6-foot birdie putt at the 13th. Fleisher then dropped one shot back at the par-3 15th.
Fleisher's tee shot at the par-3came up short of the green and plugged in a bunker. He blasted out, but his ball got caught up in the rough. He managed to get up-and-down for bogey from there to slip to 11 under.
Tewell and Fleisher both dropped their second shots within 5 feet at the par-4 16th. They both converted their birdie putts as Tewell remained one ahead of his playing partner. Tewell made a difficult up-and-down par at No. 17, while Fleisher two-putted for his par.
At the last, Tewell's second shot to the par-5 found a greenside bunker. Fleisher knocked a 3-wood onto the putting surface to set up an eagle attempt.
Tewell blasted his third shot 12 feet past the hole. Fleisher's eagle attempt narrowly missed the cup and he tapped in for birdie to join Tewell at minus-13. Tewell then drained his birdie putt for his eighth Champions Tour crown.
'Bruce and I are such good friends,' Tewell said. 'I knew it was going to be a great shoot out after the first three holes where we kind of went back-and-forth. He just gave me that little opening on the par-3 15th.'
Tewell played the event with a new putter that he received from fellow Champions Tour player Walter Hall. It worked well for him as he only needed 22 putts to complete the final round. He also played the entire 54 holes without a bogey.
'No, Walter is not getting that putter back, but I will give him one dollar for it,' joked Tewell, who became the first Champions Tour player to win without making a bogey since Morris Hatalsky won the 2003 Columbus Southern Open. 'I just putted great this week. I've been working so hard on my game and I knew if I could start making some putts that things had to turn around. I've been hitting the ball too good.'
Fleisher opened with birdies at the second and fourth to move to minus-11. He played even par over his next four holes before a birdie at the ninth got him to 12 under. It was his bogey at the 15th that cost him the tournament though.
'Doug played very well and he had to,' said Fleisher. 'I had a chance, but he got it up-and-down at 17 and 18. When you shoot 64 you're supposed to win. It was his day.'
Hatalsky posted rounds of 67-66 over the weekend to move into third place at 10-under-par 206. Jerry Pate closed with a 68 to finish one shot behind him at minus-9.
Bobby Wadkins and Joe Inman each closed with rounds of 4-under 68 to share fifth place at 8-under-par 208. Larry Nelson and Eduardo Romero, who was making his Champions Tour debut on American soil, were one stroke further back at minus-7.
Mark Lye, the first-round leader, shot even-par 72 over each of the final two rounds to finish at 6-under-par 210 in ninth place for his first top-10 finish on the Champions Tour.
Craig Stadler, the 2003 champion, shared 25th place at 1-under-par 215 after a closing 71.