Then, on the 17th, Lewis hit into the bunker off the tee and missed a long par putt to card her second bogey of the back nine.
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.
Donald, the world number six from England, recovered from a double bogey on the second with 11 straight pars from the eighth to end with a 71, while Northern Ireland's McDowell had an adventurous round of five birdies and five bogeys.
Only four off the lead after two rounds, Montgomerie followed a three-putt bogey on the long second by going in the water at the next and dropping two more strokes.
Garcia played solidly for a bogey-free 67, saving par from 18 feet on the last after driving into sand, while Kaymer recovered from a double bogey at the short second to card a 68.
Garcia wound up making a bogey on the second hole to lose the one-shot lead he had at the start of the round.
Guthrie, who fell behind for good with a bogey on the second hole, did his best to stay in range until hitting his tee shot out of bounds on the 14th for a double bogey.
But then he bogeyed 14, double-bogeyed 15 (even after laying up short of the lake on the par-five with his second shot) and finished the round with a bogey and a double-bogey for a 76 and one-under-par total.
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Fellow Ryder Cup hopeful Henrik Stenson had a double-bogey five at the second but claimed four birdies for a 70.
But the infamous shoulder-slump soon appeared as he drove through the green on the short second en route to a bogey.
The second chip, and a putt, left him with his sole bogey of the day.
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Guan had a bogey at the par-3 second after his tee shot off the back of the green.
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.
After another dropped shot at the first, a birdie two at the second was the catalyst for a recovery which saw him haul himself back to one over, but a closing bogey left him with work to do to make the cut on Friday.
After a bogey-free 68 on Thursday, he opened the second round with five birdies in nine holes and finished with a 67, good enough for the outright lead.
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But he was assessed a one-shot penalty after his second shot at the 17th hole, turning what would have been a par into a bogey.
Karlsson, chasing his second title in a week, found the trees twice as he ran up a double-bogey six.
But the seven-time major winner's bogey-free round was only good enough for second as Matthew recovered to sink a trio of birdies, including a monster 45-foot putt on the 14th.
On the par-three 17th, his first tee shot flew into the water, and although his second stuck to the green he could not make a 30-footer and came away with a double bogey.
Tees were brought forward for the second day in succession - this time by 321 yards - and Harrington took advantage by carding five birdies and one bogey.
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