Spieth's eagle at the par-5 16th hole at TPC Southwind vaulted him into the driver's seat with a bogey-free seven-under-par 63, with Cam Davis three shots behind and the best of the two Australians in the elite 70-man field.

South Korean Tom Kim is outright second following a 64, with Collin Morikawa and Argentina's Emiliano Grillo sharing third a stroke back at five-under.

“I was telling [caddie Michael Greller] maybe sometime on the back nine, I was like, I feel good, but it doesn't quite feel as tight as some of the days in the last week or so ball-striking,” Spieth told The Golf Channel.

“Then all of a sudden I ended up hitting a good one on 15 and then chipping in on 16, now it's three strokes better, so it's like, how can you not. Yeah, I think it was solid.”

Davis and Lucas Glover, who qualified for the playoffs by winning last week's Wyndham Championship, are part of a 10-way tie for fifth.

The top-70 players from the season-long FedExCup points race earned berths into the playoffs, and only the top-50 in the standings after this tournament will advance to next week's BMW Championship.

There is no 36-hole cut at the FedEx St Jude Championship for the first time.

Australian Jason Day – sitting 11th in the standings entering this week – made a solid start, carding a two-under 68 to be in the middle of the pack, joint in 34th spot.

Points leader Jon Rahm of Spain had a double bogey and four bogeys on his way to a forgettable three-over 73, finding himself in a tie for 65th.

Were he to finish the tournament that far down the leaderboard, he is still projected to hold onto the No.1 spot.

Scottie Scheffler and Rory McIlroy, entering the week second and third respectively, each shot three-under 67s on Thursday.

Spieth – the 2015 FedExCup champion – has not won on tour since the RBC Heritage in April 2022.