The duo played together in the final twosome at the East Lake golf course in Atlanta.
"I feel really good,” said the in-form Horschel. “I feel really confident with where my game is. Also, it doesn't hurt that whatever course I go to I feel like I'm going to have a good round.”
While the unheralded Horschel is the hottest player on the PGA Tour at the moment, Kirk is no slouch either.
Kirk is currently number one on the FedEx Cup standings after winning the Deutsche Bank Championship two weeks ago.
"Definitely nice to get off to a good start," said Kirk. "The swing felt really good."
Any of the current top-five players on the points list would capture the FedEx Cup with a win this week. Anyone outside the top five will need a win and some help from others.
Masters champion Bubba Watson, who is number three in the standings, is tied for third at three-under 67. He is joined there by Australia's Jason Day, Patrick Reed and Jim Furyk.
Cameron Tringale, Zach Johnson, Bill Haas and Matt Kuchar are tied for seventh at two-under 68.
World number one Rory McIlroy is three shots adrift of the leaders at one under with Ryan Palmer, Sergio Garcia, Adam Scott and Rickie Fowler.
Rounding out the top five on the points list entering this week was The Barclays winner Hunter Mahan.
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