Fitzpatrick hit 9-iron to within one foot on the famed, closing lighthouse hole to set up the winning birdie on the famed Pete Dye layout he played as a child on vacation.

Fitzpatrick had to sweat out a couple of prime chances by Spieth on Sunday's first two extra holes. Spieth raised his putter in triumph before watching his 12-foot birdie putt catch the right edge and spin out the first time the pair played the 18th. Then Spieth's nine-foot birdie attempt ran out of steam on the right edge at the 17th hole.

There was no doubt about the final hole as Fitzpatrick, from 187 yards out, hit the front of the green and watched the ball settle next to the hole. Spieth's attempt from 26 feet away rolled past and Fitzpatrick tapped in for the victory.

"It's hard to describe," Fitzpatrick said. "It doesn't get better than this."

Fitzpatrick won US$3.6 million (A$5.4m) from the elevated purse of US$20 million in the sixth designated event of the year on the PGA Tour, topping a field that included seven of the world's top-10 players.

Fitzpatrick trailed Spieth by two with five holes to play, but made birdies on the 15th and 16th to tie for the lead at 17-under.

Cam Davis finished in a share of seventh at Harbour Town. PHOTO: Andrew Redington/Getty Images.

The third-round leader by one shot, Fitzpatrick closed with a three-under 68 and matched Spieth (66) at 17-under.

Patrick Cantlay (68), grouped with Fitzpatrick and Spieth, was third at 16-under. Cantlay lost this title in a play-off to Spieth a year ago.

"Another close call here," Cantlay said.

Xander Schauffele (66) finished fourth at 15-under. Sahith Theegala (65) and Hayden Buckley (67) tied for fifth at 14-under, and Brian Harman (67), South Korea's Sungjae Im (67), Argentina's Emiliano Grillo (68) and Australia's Cam Davis (68) finished 13-under in a tie for seventh.

Fellow Aussies Adam Scott (72) finished seven-under and Lucas Herbert (74) was even par for the event.

"It's hard to describe. It doesn't get better than this." - Matt Fitzpatrick.

Scottie Scheffler (70), No.2 in the world, was tied for 11th at 12-under, while Masters champion Jon Rahm (68) was equal-15th.

Spaniard Rahm will take a break and return in two weeks to defend his Mexico Championship title.

Fitzpatrick carried a one-shot lead into the final round, but watched as Spieth made four birdies in the opening six holes to share the lead.

Spieth made bogey on the par-3 14th while Fitzpatrick answered with consecutive birdies on 15 and 16.

The pair remained tied until Fitzpatrick's remarkable approach on the winning hole.

Cantlay, so close a year ago, lost his chance at victory with consecutive bogeys on the 13th and 14th holes.