Rory McIlroy trails in FedEx Cup race at Tour Championship
|Tour Championship first round (US unless stated)|
|-4 Billy Horschel, Chris Kirk, -3 Bubba Watson, Patrick Reed, Jim Furyk, Jason Day (Aus), Selected others: -1 Rory McIlroy (NI), +4 Hunter Mahan|
Rory McIlroy remained in contention for the PGA Tour's FedEx Cup title after finishing three shots adrift after the first round at the Tour Championship.
Billy Horschel and Chris Kirk moved ahead in the race for the £6m bonus as they shared the lead on four under par.
Bubba Watson was on three under with McIlroy two shots further back in the Tour finale at East Lake in Atlanta.
Victory for either McIlroy, Horschel, Kirk, Watson or Hunter Mahan would secure the season-long title and bonus.
|FedEx Cup top five after three of four events|
|1. Chris Kirk (US)||4,314 points|
|2. Billy Horschel (US)||4,305|
|3. Bubba Watson (US)||4,058|
|4. Rory McIlroy (NI)||3,735|
|5. Hunter Mahan (US)||3,363|
Northern Irishman McIlroy, 25, had four birdies and three bogeys in his opening round as he went out in 34 and came back in 35.
"Regardless of whatever is at stake in terms of prize money, I would just like to win the FedEx Cup," said the world number one.
"With the year that I've had I really want to finish it off well."
In theory, anyone left in the field can win the season-long title with victory at the Tour Championship but those outside the top five in the standings will have to rely on other results.
US Ryder Cup players Patrick Reed and Jim Furyk were also at three under alongside Australian Jason Day, while England's Justin Rose finished two over and Germany's Martin Kaymer three over.
Fifth-ranked Mahan's bid for the crown looked to be over as he shot a four-over 74 in a tie for 27th.
Tournament officials announced that Friday's second-round tee times will be brought forward by three hours because of a bad weather forecast.