The decision was great news for Scott Stallings, who made a quadruple-bogey 8 on the third hole and already was 7-over par after four holes.
But on 16 a wayward tee shot resulted in his third bogey of the day.
Phil Mickelson recovered from a double-bogey six at the third hole to also post a 69, leaving him one shot adrift of McDowell and tied for third with compatriot Steve Stricker (69).
He three-putted from about 15 feet for bogey on third, made bogey with a wedge in his hand on the par-5 fifth hole and stumbled to a 76.
The 30-year-old McDowell is full of confidence after winning the Wales Open earlier this month and fired six birdies to reach four under before his third bogey on his final hole trimmed his lead to two going into the weekend.
BBC: US Open: Graeme McDowell leads as Phil Mickelson surges
Drummond, ranked 717th in the world, birdied four consecutive holes from the third, and then made amends for a bogey at 14 by picking up shots at his next three greens to finish on 64.
Ireland's Harrington, bidding for a third straight major, ran up a quadruple bogey nine on the par-five second and did well to recovery his composure to finish with 73 for one-under, while Garcia, still seeking his first major carded a three-over 75.
Luke Guthrie, tied with Thompson for the 54-hole lead, fell behind with a bogey on the second hole and closed with a 73 to finish third.
Kaymer sank seven birdies and just one bogey to seal his third straight strokeplay victory, following his victories at the USPGA Championship in August and September's KLM Dutch Open.
Angela Stanford also had a share of third until the final hole, when she lipped out a short par putt for her first bogey in a closing 69.
However, it was an encouraging start after the US Open, where he made double bogey on the final hole when a par would have been good enough to win his third straight major.
He was leading the U.S. Open at Winged Foot, looked like he was going to win a third straight major championship, then had a complete meltdown on the 18th hole there, made a double bogey, lost and has not contended in the two majors since.
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