Horschel is coming off his best finish on the tour, a tie for seventh place at the Frys.com Open. He backed up that showing with his second straight 64 Friday and his 12-under 128 total gave him a two-shot lead over Webb Simpson and Michael Thompson.
Simpson is involved in what could turn out to be a dramatic race for this year's money crown. He needs to finish among the top 15 this week to take over the No. 1 spot on the money list from Luke Donald.
Donald, ranked No. 1 in the world, is trying to become the first player ever to be the leading money winner in both the United States and Europe in the same year. He is not playing this week, but has committed to play next week in the season finale at Walt Disney World.
Simpson, who shared the lead after the opening round, shot a 67 at the Sea Island Resort's Seaside Course Friday and Thompson had a 65 to leave them tied at 130.
Former British Open champion Louis Oosthuizen and Nick O'Hern were tied for fourth at 132. Bryce Molder, who recorded his first tour win last week at the Frys.com Open, was in a tie for eighth at 135.
It took a score of 1-under 139 to make the cut and Bud Cauley was among those surviving to the weekend.
Cauley, who turned professional in June, finished 36 holes at 4-under 136 and thus virtually wrapped up a spot among the top 125 money winners this year. By doing so he will become just the sixth player in PGA Tour history to earn full-time playing privileges without going through the qualifying tournament.