AP photo by Mark Humphrey / Brooks Koepka tees off on the 18th hole at TPC Southwind during the first round of the World Golf Championship-FedEx St. Jude Invitational on Thursday in Memphis.

MEMPHIS — Brooks Koepka has been working hard to fix his golf game even with an injured left knee.

The four-time major champion put everything together Thursday.

Koepka matched his career best with an 8-under-par 62 to take a two-stroke lead over Rickie Fowler and Brendon Todd after the first round of the FedEx St. Jude Invitational. Koepka won the event last year — its debut as a World Golf Championship — for the most recent of his seven PGA Tour victories.

Six days after missing the cut at the 3M Open at TPC Twin Cities in Minnesota, Koepka had nine birdies and a bogey on a windy day at TPC Southwind, which long hosted the FedEx St. Jude Classic before last year taking the WGC slot previously held by Firestone Country Club in Akron, Ohio. Koepka, No. 6 in the current Official World Golf Ranking, credits all his work with coaches Claude Harmon and Pete Cowen rather than how comfortable he is on the Memphis course.

"It's the first time I feel like I know where my misses are, I know when the club's in the correct spot, I know when the putting stroke's nice," the 30-year-old Koepka said. "It's all just the work we've put in over the last three weeks of countless hours of beating balls and on the putting green."

Koepka is also the reigning winner of the PGA Championship, the 2020 edition of which will be staged next week at TPC Harding Park in San Francisco, and his coaches helped him realize last weekend how much he was favoring his injured left knee.

He sure found something at TPC Southwind, where he turned in his lowest round of the year and best in seven events at this course. This was his eighth time shooting under par in his past nine rounds here. He needed 26 putts after working earlier this week with Phil Kenyon.

"It's nice to see those results and all the changes we've made come through," Koepka said.

The 32nd-ranked Fowler chipped in from 30 feet to tie Koepka, then two-putted for bogey after putting his tee shot in the left rough on his final hole to finish at 64.

It was Fowler's lowest round after missing three cuts since the PGA Tour's return to play in June. He credited getting back to playing golf instead of just working on the range.

"I was able to get a lot of good work with the putter last week and get myself back into some better positions to free up the putter," Fowler said. "I was pulling a lot of putts, I was kind of tense with it, so it's nice to see things kind of pay off."

Fowler and Todd, who had one bogey in a seven-birdie round, were a shot ahead of South Korea's Sung Kang. Justin Thomas, who can move back to No. 1 in the world with a victory, shot a 66 and was tied with Max Homa, Matt Kuchar and Chez Reavie for fifth.

Young Spanish star Jon Rahm, playing his first event since moving to the top of the ranking with a win at the Memorial Tournament on July 19, shot 70. He said he didn't play like a No. 1 golfer, especially on a day when players could lift, clean and place on perfect greens.

Rahm said Koepka and Fowler made it look easy, though.

"I just didn't have a swing today," Rahm said. "I felt terrible, I could not figure out where the ball was going to go, and I was just trying to minimize mistakes."

Rory McIlroy, who also could move back to No. 1 with a win, opened with a 73 and was tied for 67th in the 78-player field.

Koepka, teeing off in the penultimate group, birdied his first four holes to shoot up the leaderboard and went to 4 under to share the lead with Bryson DeChambeau after rolling in a 23-footer for birdie on the par-3 No. 4.

Koepka made a bogey on No. 7 after he missed a 10-foot putt. He hit his tee shot on the par-3 eighth hole within seven feet of the cup and made the birdie putt. He used his putter again for a second straight birdie, rolling in an 18-footer on No. 9 to make the turn with the best first nine of the day, a 30 that had him tied for the lead at 5 under.

Then he just kept making birdies. He added two of his first four on the back nine, the second when he sank a seven-footer on No. 13. He tapped in for a birdie after just missing an eagle on the par-5 No. 16.

Tee times for the second round have been moved to the morning to avoid predicted thunderstorms.

With no fans allowed at an event that has benefited St. Jude Children's Research Hospital since 1970, St. Jude had to get creative to raise both money and attention for a hospital that never bills its patients.

Fans and pros can take part in Birdies for St. Jude making either a one-time donation or pledge an amount for each birdie by a specific player during this tournament. Artwork from St. Jude's patients also has been turned into caddie bibs and special shoes for both DeChambeau and Fowler that will be auctioned off for the hospital.