Hearn fired a four-under 68 on Saturday to move to 15-under, two shots clear of Jason Day and Bubba Watson.
Michael Putnam and Brooks Koepka are a further shot back on 12-under, with a group of five players on 11-under.
Hearn, who is looking for his first win in 166 PGA Tour events, took the turn in 33 after birdies at the second, seventh, and eighth holes, before picking up further shots at the par-3 15th and par-5 16th. A birdie at the penultimate hole was the only mark on his card.
A second-round 66 shot Day into contention, and the Australian remained in the hunt with a three-under round of 69 that included two bogeys, a double bogey and a massive seven birdies – including six in seven holes from the 12th to the 18th.
Watson, who joins Day on 13-under, also endured somewhat of an up and down round, with three bogeys intermingled with five birdies and an eagle at the par-five 13th.
Putnam and Koepka carded rounds of 68 to sit three shots off the pace, while overnight leader Chad Campbell is one of those on 11-under. The American managed just one birdie on the day, a four at the par-five 17th, while registering a bogey at the third and a disastrous triple bogey at the par-four 11th.