Purtzer closed with 4-under-par 68 to match Roberts (69) at 10-under 206 on the Valencia Country Club course. The 55-year-old Purtzer earned $240,000 for his fourth career victory on the 50-and-over tour.
"I feel really fortunate, and lucky," said Purtzer, in tears after the victory.
After the two players matched pars on their first three trips down the par-5 18th in the playoff, Purtzer finally broke through with his 17-foot birdie putt. Roberts had a chance to match, but his 12-foot birdie try just missed on the right side.
Purtzer credited fellow tour player Dave Stockton for some recent putting help.
"I have to thank Dave. He gave me a lesson and I putted better," said Purtzer, a five-time winner on the PGA Tour. "I hung in there and played really well."
Purtzer, playing in the group in front of Roberts, made a 10-foot birdie putt on No. 18 in regulation to reach 10 under. Roberts, hampered by back problems, had a chance to win in regulation, but missed a 19-foot birdie try.
"It was a long day," Roberts said. "We took it as far as we could go. I was just happy to get in a playoff. My back has been really bothering me."
TCL CLASSIC: In Sanya, China, Chapchai Nirat of Thailand shot 1-under-par 71 to claim his first victory in a major tournament.
The 23-year-old led from the start and finished with a 22-under 266 at the Yalong Bay Golf Club on Hainan Island. Rafael Echenique of Argentina finished three strokes back after 68. Prayad Marksaeng of Thailand was two more shots behind after 67.