MEMPHIS, Tenn. -- When Justin Thomas needed an emergency fill-in caddie on Tuesday, he called Jim "Bones" Mackay, one of the best to ever carry a bag on the PGA Tour.
Thomas could have never imagined the circumstances in Sunday's final round of the WGC-FedEx St. Jude Invitational, when he was paired in the third-to-last group with Phil Mickelson, who was Mackay's boss for 25 years, when they worked together to win 42 times, including five majors.
Thomas came from four shots back to win for the third time this season, shooting a 5-under 65 to beat Brooks Koepka, Daniel Berger, Tom Lewis and Mickelson by three shots at TPC Southwind.
The victory matched the largest final-round comeback of Thomas' career and it will move the 27-year-old back to No. 1 in the world, a ranking he held for four weeks in 2018.
"It was a hard-fought day, but it meant a lot I just felt like [because of] how we did it, being four behind to start the day," Thomas said. "I haven't exactly played well coming from behind in the past and I feel like I learned a lot from that. Especially at Colonial, I just didn't handle it well and I pressed way too hard and I definitely learned a lot from Workday and Columbus, (Ohio), just how I handled that lead and that situation the last couple holes. I did obviously a very poor job, but because of that I felt a lot more calm and stayed more in the moment today."
It certainly helped having a caddie as experienced as Mackay with him.
"He did great," Thomas said. "He did what a lot of great caddies do and they let the player do it. There's a reason why they're the best and he did an unbelievable job this week, especially for being in that situation, which we had never been in that together."
Thomas' longtime caddie, Jimmy Johnson, fell ill during the third round of the Memorial two weeks ago. Thomas' dad and swing coach, Mike, was in the gallery and took over his bag.
Johnson, who has been working with Thomas since 2015, was expected back at this week's tournament, but he was still feeling dizzy, so Thomas called Mackay for help.
Mackay will also be on Thomas' bag for this week's PGA Championship at TPC Harding Park in San Francisco. Thomas said Johnson was still undergoing medical testing.
"The big thing is he's fine when he's inside and not in the severe heat like that," Thomas said Saturday. "The big thing is we're just trying to figure out what it is or if there's something in particular, whether it's some medicine he takes. More often than not, it's not something crazy major.
"I love Jimmy to death, he's part of the family and I want him over anybody else in the world on my bag for the rest of my career, but if it means his health, then that's what it is."
Mackay, who works as an on-course reporter for the Golf Channel, won't start working tournaments until the FedExCup Playoffs start later this month. He came out of retirement as a caddie last month when Englishman Matthew Fitzpatrick's regular caddie decided not to travel to the U.S. because of the coronavirus pandemic. Mackay worked back-to-back tournaments at Muirfield Village.
Despite what might have been uncomfortable circumstances, Mickelson, 50, played well in the final round while trying to become the first player 50 or older to win a World Golf Championship event. His 3-under 67 helped him secure his highest finish in a Tour event since he won the AT&T Pebble Beach Pro-Am in February 2019.