The American had shared the lead with compatriot Chris Kirk heading into the second day of the PGA Tour's season-ending event in Atlanta, but demonstrated consistency to move two strokes clear of the chasing pack.

The threat of stormy weather meant play got under way early, and Horschel had to work harder for his score than he had on day one.

In contrast to Thursday when he produced a flawless display to share the lead, Horschel hit two bogeys during his 18 holes on Friday.

A 25-foot putt for birdie on the 14th was one of the highlights of Horschel's round, while an excellent approach shot on the 17th ensured he ended the day two clear.

The in-form Rory McIlroy moved into second place with the best round of the day - a five-under 65.

The Northern Irishman hit six birdies on his way around East Lake Golf Course, with a mammoth putt on the last completing a fine display.

There was a moment of light relief for McIlroy on the 14th when his ball struck a tree before landing in the pocket of a spectator. The world number one was subsequently granted free relief and went on to make par.

"It hit up on the tree and ricocheted straight into his pocket," McIlroy said. "I wasn't going in there. I know how sweaty my pockets are. I'm not going into anyone else's."

Kirk followed up his leaderboard-topping performance on day one by delivering a two-under round of 68 on Friday.

The American now sits level with McIlroy and Australia's Jason Day, who managed a second consecutive 67.

Kevin Na and Justin Rose were among the biggest movers of the day, both carding rounds of 66 to move into shares of fifth and 10th respectively.