Rounds of 64, 69 and 67 through the first three rounds at Firestone Country Club leave him in good position to win his first trophy of the year on Sunday.
There's just one problem: He'll be sharing a tee box with Tiger Woods.
Woods recovered from an even-par round on Friday to shoot 5-under on Saturday, climbing the leaderboard to second place heading into the final round.
Woods was 1-under through the front nine, but birdied Nos. 13, 15, 16 and 18 to finish with a 5-under 65.
Harrington, after posting the best score of the weekend thus far on Thursday, also had four birdies on the back nine but bogeyed the par-5 16th to finish at 3-under for the day.
South African Tim Clark shot 1-over on Saturday to fall into third place, tied with Jerry Kelly at 5-under. Ten players sit at 4-under, while five more are at 3-under heading into the final round.