Jordan Spieth made a late eagle to take the first round lead at the rain-soaked FedEx St Jude Championship, with Davis well placed in equal fifth just three shots behind at TPC Southwind in Memphis, Tennessee.

The top-70 players from the season-long FedExCup points race earned berths into the $US75 million ($A115 million) playoffs, and only the top-50 in the standings after this tournament will advance to next week's BMW Championship.

The 28-year-olds impressive round featured three birdies, an eagle and just one bogey which lifted the 2017 Australian Open champion from 66th to 48th in the live points standings, good enough to advance to next week if he can stay there.

"I played really well today. For the most part it was pretty stress free," Davis said.

"I kept it in the fairway pretty well, which was important, because anything that missed the fairway was just going to get covered in mud and be tough to control.

"I did a good job there and stayed patient and cruised around, then I had a little hot stretch on the back nine there for a few holes. It was a good start to the tournament.

"More of that would be good."

After teeing off on the 10th, Davis rocketed up the leaderboard with a birdie on his 11th hole of the day, then an eagle followed by a chip-in birdie to pick up four shots in three holes.

"I played really well today. For the most part it was pretty stress free."- Cam Davis. 

"It was just very nice to get a little bit of momentum and get the score going from one-under to four or five-under pretty quick there," he said.

Jason Day, the only other Australian in the field and sitting 11th in the standings entering this week, also made a decent start with a two-under 68 to be joint 34th.