- US Open champ shares first-round lead
- Top-five finish could land $10M prize
- McIlroy recovers from shaky start
(CNN)He's a man on the verge of winning $10 million -- but Dustin Johnson says he feels "really relaxed."
The FedEx Cup leader had a share of the lead at the Tour Championship after Thursday's opening round in Atlanta and will claim the huge prize if he wins, although a top-five finish could well be sufficient.
Johnson, along with Japan's Hideki Matsuyama and fellow American Kevin Chappell, carded four-under-par 66 at East Lake, with Northern Irishman Rory McIlroy -- who must win to stand any chance of scooping the jackpot -- two shots further back.
With the Ryder Cup looming, US Open champion Johnson is one of eight members of the American team on display and he is looking sharp as the countdown to the contest gathers pace.
"There's a lot on the line -- but there's a lot on the line every week," he told reporters.
"I felt really relaxed out there all day."
McIlroy -- who recovered from a shaky start to hit seven birdies and keep himself firmly in the picture -- told UK broadcaster Sky Sports it would "be OK for the Ryder Cup next week playing like this."
The 27-year-old, who has been frustrated by erratic putting in recent weeks, said that aspect of his game remained a work a progress.
"I'm just happy that the score at the end of the day is under par," said the four-time major winner, who was tied for seventh in a group of four players including US rival Jordan Spieth.
"I am mixing a lot of bad with a lot of good so if I can eradicate the mistakes, seven birdies around this golf course is pretty good.
"As you can see, once I get a couple of birdies I get some confidence and momentum.
"I am actually more comfortable on the breaking putts. The ones that I missed were dead straight."
World No. 1 Jason Day, one of five players in the 30-man field guaranteed to claim the $10 million jackpot with victory Sunday, was tied for fourth on 67.
England's Paul Casey was in the group on 68 while Australia's Adam Scott was a shot back tied 11th and US Ryder Cup player Patrick Reed was equal 18th on 73.
Two-time Masters champion Bubba Watson, who is hoping to win the final US Ryder Cup wildcard place but has slim hopes of taking the jackpot, shot 72.