Last-start winner Billy Horschel is surging but Jason Day remains in the hunt in the third round of the PGA Tour Championship.

American Billy Horschel remains in control of both the tournament and the season-long FedEx Cup midway through the third round of the US PGA Tour Championship but Jason Day remains in the mix.

Day was even par on his third round through nine holes to remain six under for the tournament, good enough to be in third place but five shots off the pace.

Horschel extended his overnight two-shot buffer with three birdies in his opening nine holes, sending him to 11 under and three clear of world No.1 Rory McIlroy, who was alone in second place.

Both Horschel and McIlroy had destiny in their own hands in terms of winning the season-long points race and the $US10 million ($A10.82 million) bonus, knowing a win at East Lake Golf Club on Sunday will ensure a double celebration.

Day opened his third round in style with a birdie but gave the shot right back with a bogey on the second.

Another birdie on the third hole kept him on the pace but a bogey on the seventh, after missing a short par putt, stalled the Queenslander once more.

World No.2 Adam Scott was trying to make a move up the boards, four under on his round through 14 holes to be three under for the week and tied ninth.

John Senden sat in the clubhouse with a one-under 69 leaving him tied 22nd at six over while Geoff Ogilvy continued to occupy the last spot in the 29-man field, now at 17-over after a 73.