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.''
Horschel won the BMW Championship last week and was seen sprinting off the fairway toward a portable toilet because he couldn't hold it anymore. Kirk won the Deutsche Bank Championship the previous week, and he surprised even himself when twice - a career high - he pumped his fist after making a putt.
They 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.
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.
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.
DIVOTS: Jason Day's coach and caddie, Colin Swatton, had to stop after eight holes when his back locked up on him. Day's mental coach caddied the rest of the way. ... Cameron Tringale made his debut in the Tour Championship with a 68, but it wasn't his first time at East Lake. He played at Georgia Tech, which has a corporate membership at East Lake. ... The average score was 69.8.