The Spaniard, 24, holds a three-shot lead after a bogey-free round at the TPC Southwind course in Memphis.

Bubba Watson is among a group of five on five under in the third of this year's four World Golf Championships.

Ian Poulter and Tyrrell Hatton are a shot further back, while world number three Rory McIlroy birdied four of his last five holes to finish on one under.

Over the past two months, world number eight Rahm has also finished in a tie for third at the US Open and tied-second at the Andalucia Masters, and had to cut short a practice round on Wednesday due to jet lag.

"I tried to put a little emphasis on being rested and it paid off today," said Rahm. "I have three more days to go and hopefully the battery lasts.

"My last 10 holes, I made a lot of putts. I read the greens properly, but I think the main key out of all that, it was my positioning."

Justin Rose is on three under while fellow Englishman Tommy Fleetwood, who came second at last week's Open, is on two under with American world number one Brooks Koepka.

Rahm, who was tied for 11th at the Open, needed just 23 putts as he seeks his third title of 2019 having won the Irish Open and the Zurich Classic of New Orleans team competition with America's Ryan Palmer.