ATLANTA — Chris Kirk and Billy Horschel have little in common except a clean card of 4-under 66 at the Tour Championship and their chances at the biggest payoff in golf.

Kirk and Horschel, the top two seeds going into the FedEx Cup finale at East Lake, played in the final group and traded birdies — neither of them made a bogey — over four hours in steamy weather to share the lead.

They need only to win the Tour Championship to claim the FedEx Cup and its $10 million bonus.

“Billy has obviously been playing some pretty incredible golf with winning last week and finishing second the week before,” Kirk said. “And I’ve been doing all right myself.”

Kirk is a 29-year-old who went to Georgia and plays golf without a pulse. Even when he chipped in from 80 feet on the 17th hole, he simply smiled and bowed his head before slapping hands with his caddie.

Horschel is a 27-year-old who went to Florida, brash enough to wear octopus prints on his pants in the final round at a U.S. Open, to flip his cap around backward and to pump his fist for routine pars.

They grew up playing amateur golf against each other. They were teammates in the Walker Cup.

And they are leading the race to the FedEx Cup.

“We’re probably two completely opposite people in the sense that he just looks like he’s moving very slow and nothing affects him,” Horschel said. “I look like I’m running around the golf course — literally last Sunday. But Chris and I get along very well. We seem to always play well when we’re paired together.”

The pair didn’t have the course to themselves.

Masters champion Bubba Watson made seven birdies to offset a few mistakes, such as trying to hit a shot through a gap in the trees. It worked at Augusta National two years ago. His ball clipped a branch Thursday, leading to double bogey.

A bogey from the bunker on the par-3 18th hole gave him a 67, leaving him in reasonable shape. Watson was tied with former University High golfer Patrick Reed, Jim Furyk and Jason Day.

The top five seeds need only to win the Tour Championship to claim the FedEx Cup. Watson is third.

Rory McIlroy is at No. 4, and he didn’t hurt himself. McIlroy wasn’t at his best, though he made enough birdies and key par saves for a 69 that kept him very much in the hunt.

“You can really shoot yourself out of it,” McIlroy said. “Even though I didn’t play great, I kept it together.”

Hunter Mahan might have shot himself out of it. Mahan is seeded fifth and opened with a 74. Only one other player in the 29-man field — Geoff Ogilvy, who is just happy to have made it to the Tour Championship — had a worse score.

Mahan, one of three captain’s picks for the Ryder Cup, has broken par once in his last nine rounds since winning The Barclays.

Kirk was left off the Ryder Cup team, even though he has two wins this season and had just won a FedEx Cup playoff event the day before U.S. captain Tom Watson announced his three picks. Horschel might be the hottest in golf at the moment. He is prone to go on big streaks like this.

They have only one cup in mind, and they took a big step toward it Thursday.

“This is my sixth week in a row. I haven’t played more than three events in a row this year,” Horschel said. “But I have no issues with that. Listen, this is the FedEx Cup playoffs. If you can’t get yourself in shape and get up for it on a daily basis, they why are you playing this game?

“It’s our playoffs. It’s like the World Series or the NBA championships and NFL playoffs. And we’re all tired and we’ve just got to figure how to put it out of our mind and go out and play golf.”

Horschel has more on his mind than just a $10 million bonus. His wife is expecting their first child in two weeks. He said even if she were to go into labor early, they agreed that he should stay in Atlanta and try to win the FedEx Cup.