Late birdie lifts Chris Kirk to PGA victory at Sentry tournament
Kirk's eight-under-par 65 gave him a 29-under total of 263 on the par-73 Plantation course, where he started the day with a one-stroke lead.
He had pushed the advantage to two strokes with five front-nine birdies.
But Theegala's run of four straight birdies from the 13th through the 16th had seen him take the solo lead before Kirk responded at the par-five 15th - where he had a 12-foot eagle chance but settled for a birdie.
Theegala, who had 10 birdies in his 10-under par 63, was in the clubhouse on 28-under 264 when Kirk birdied 17, landing his 206-yard second shot two and a half feet from the pin.
"That one on 17 I'll remember for a long time," said Kirk, whose par at the par-five 18th was plenty to claim a sixth PGA Tour title.
Kirk had qualified for the elite season-opening event - open to last year's tournament winners and the top 50 in the FedEx Cup - with a victory last year at the Honda Classic, where he ended a near eight-year title drought.
"It's unreal," Kirk said. "It's just so unexpected. I had a really great off-season and I got a lot of good work done and felt good about the year, but you never really expect to go shoot 29-under.
"It's unbelievable - still kind of soaking it in," he said.
Theegala, who won his maiden title in Napa, California, in September, started the day three back and started applying pressure early.
He birdied four in a row from the second through the fifth and added birdies at nine and 11 before his four-birdie burst on the back nine.
He missed the green at the 17th, and while he rolled in a nine-foot par-saving putt he couldn't take advantage of the par-five 18th.
On another low-scoring day, three-time major winner Jordan Spieth was tied for the lead at 27-under through 15 holes, but a tough lie near the lip of a fairway bunker at 16 led to a bogey and his eight-under par 65 left him alone in third on 265.
Stung by his inability to take advantage of birdie chances at the fifth and sixth, Spieth birdied five in a row from the seventh through the 11th before back-to-back birdies at 14 and 15.
Spieth admitted that he would have expected an eight-under round would be good enough to at least get in a playoff.
"There was just some phenomenal golf played everywhere," he said.
South Korea's An Byeong-hun carded a 66 for solo fourth on 266.
Compatriot Im Sung-jae headed a group on 267 after a 10-under 63 that included 11 birdies and gave him 34 birdies for the week - the most ever in a 72 hole PGA Tour event.