A pair of PGA Tour veterans, Brian Bateman and Tom Pernice, Jr., share fourth place at 4-under-par 68. Rich Beem, the 2002 PGA Championship winner, is alone in sixth place at minus-3 at Jungmun Golf Club.
Purdy got into red figures as he birdied the par-5 third to get his round going. At the next par-5, No. 7, Purdy again posted birdie and came right back with a birdie at eight to get to minus-3.
The 31-year-old Purdy again took advantage of a par-5 to get to 4 under, as he birdied the par-5 11th. The University of Arizona graduate dropped in a birdie attempt at 12 to make it two straight.
Purdy, who owns one win on the Nationwide Tour, converted a birdie try at the 14th to move to minus-6. He again birdied a par-5, the 15th, to move out in front at 7 under. He parred his final three holes to remain there.
Harrington, who owns nine wins on the European Tour, got off to a quick start with birdies at the first and third. He faltered to a bogey at the fifth, but bounced right back to birdie six to get back to minus-2.
The Irishman followed his birdie with an eagle at the par-5 seventh. He parred seven straight holes around the turn before a birdie at the par-5 15th moved him to 6 under. Like the leader Purdy, Harrington parred his final three holes.
Yang tripped to a bogey at the second, but bounced back with a birdie at six and an eagle at seven to jump to minus-2. The Korean birdied the 11th and carded another eagle, at the par-5 15th, to move to 5 under. He birdied No. 16 before carding pars at the last two holes.
Mark Calcavecchia, Harrison Frazar, Trevor Immelman, Miguel Angel Jimenez, Zach Johnson, Kevin Na and Tim Petrovic share seventh place at 2-under-par 70. John Rollins, Daniel Chopra, Carlos Franco, Shaun Micheel and Joe Ogilvie are one stroke further back at minus-1.