ATLANTA, September 12, 2014 (AFP) - The duo played together in the final twosome at the East Lake golf course in Atlanta and as the co-leaders they will be paired again in Friday's second round.
"I feel really good. I feel really confident with where my game is," said the in-form Horschel, who shot his ninth straight round in the 60s. "Also it doesn't hurt that whatever course I go to I feel like I'm going to have a good round.
"I'm just in the mindset that I'm going to find some way to put a good round together and that's the way it's been the last nine rounds."
While the unheralded Horschel is the hottest player on the PGA Tour at the moment, Kirk is no slouch either.
Kirk is currently No. 1 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 today."
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.
McIlroy, this year's Open champ and PGA Championship winner, is fourth on the points list.
Rounding out the top five on the points list entering this week was The Barclays winner Hunter Mahan, who is well off the pace in a tie for 27th after the first round in Atlanta.
Kirk made the turn with six straight pars from nine, but he closed out his round in impressive fashion.
Following his third birdie on an 11-foot putt on No. 15 and a par on 16, Kirk played a brilliant chip shot just off the green and rolled his ball in from 82 feet away for birdie to join Horschel at four under.
"Nice lie there in the fairway," Kirk said. "Very great shot, but to see it going in was pretty cool."
Another pair of pars preceded Horschel's fourth and last birdie of the day at the par-four 16th. He made a putt outside of 10 feet to take the outright lead at 16 but Kirk joined him on the next hole.
Leading scores after Thursday's first round of the Tour Championship (par 70, USA unless noted):
66 - Billy Horschel, Chris Kirk
67 - Jim Furyk, Bubba Watson, Jason Day (AUS), Patrick Reed
68 - Matt Kuchar, Zach Johnson, Bill Haas, Cameron Tringale
69 - Sergio Garcia (ESP), Adam Scott (AUS), Ryan Palmer, Rory McIlroy (NIR), Rickie Fowler
70 - Kevin Na, Brendon Todd, Morgan Hoffmann, Russell Henley
71 - Gary Woodland, Jordan Spieth, Hideki Matsuyama (JPN)
72 - John Senden (AUS), Justin Rose (GBR)
73 - Jimmy Walker, Martin Kaymer (GER)
74 - Hunter Mahan, Webb Simpson
77 - Geoff Ogilvy (AUS)