Rahm clinched the title with a putt from 66 feet on the same 18th hole to win about half an hour after Johnson amazing effort.

"I was kind of laughing at my putt because it was one of those where you, you're not really thinking you're going to make it," Johnson said after a closing 67 saw him locked with Rahm on four-under-par 276.

"I played an unbelievable putt, got in the playoff and then Jon made an even more ridiculous putt on top of me."

Johnson had the opportunity to force a second play-off hole but his long birdie putt was inches away, ensuring that Rahm would enter next week's Tour Championship in Atlanta second in the FedEx Cup points standings behind the American World No.1.

"I gave it a good effort," Johnson added of his second long putt in a matter of minutes on the 18th hole.

"I played an unbelievable putt, got in the playoff and then Jon made an even more ridiculous putt on top of me." – Dustin Johnson

"I thought I made it when it was coming down the hill. It just kind of ran out of a little bit of speed and missed just low."

Johnson said he felt in good shape heading into the Tour Championship next week with a US$15 million (AU$20m) prize on offer for the winner.

"Obviously the goal was to be No.1 going into East Lake, and I am still No.1. I like my position," he said.

"I feel like the game is in really good form right now. I'm really, really pleased with all aspects of the game."

A runaway winner last week in Boston, the in-form Johnson will start the 30-man Tour Championship as the top seed at 10-under with World No.2 Rahm at eight-under.

Behind them will be Justin Thomas (-7), Webb Simpson (-6) and Collin Morikawa (-5) while another 20 players will start at one-under – Australia's Marc Leishman among them – or better.

Queenslander Cameron Smith is among five players to commence the US$60m (AU$81.5m) tournament at even par.