"Really, the letdown, I hit a terrible shot on 17," Couples said. "I smoked it down there and I just tried a shot that maybe I'm not accustomed to. Under that kind of pressure, I think it was a mental blunder to not hit the green there and not make birdie."
Cook beat Frost in a playoff at the season-opening Mitsubishi Electric Championship in Hawaii, and made a final-round run at Shoal Creek.
Cook birdied five of the first eight holes and finished his round with a 66 a couple of minutes before the weather delay.
"We were running all day trying to outrun it," Cook said. "I guess this is the very definition of scattered thunderstorms. They were all over the place."
Langer closed with a 63 to tie the course record with Gary Player from the 1984 PGA Championship. Langer had 10 birdies and finished at 10 under.
"If I had made some putts earlier in the week," he said, "it would have been pretty nice."
Two-time defending champion Tom Lehman tied for 22nd at 6 under.