Couples had seven birdies in a round of 64, while Peter Fowler finished with a 65 to trail the leader by two shots.
"I'm just happy to be in the hunt with 18 holes to play," said Langer, who won this title two years ago at Carnoustie. "There's a lot of guys in with a chance, so I know I'll have to shoot one of the lower scores to win."
Couples had birdies at Nos. 1, 3 and 7, 8, 9. On the back nine, he had two birdies and dropped one shot at the 14th.
"It was a very, very good day," Couples said. "Once I got going, I didn't miss many shots. I hit my irons solid and when I hit them close, I made the putt."
On a day of low scoring despite strong wind and some showers, Langer and Couples each reached the turn in 30.
Langer birdied the first, fourth, sixth and ninth, eagled the long seventh and dropped a stroke at the fifth. He had nine straight 4s on the back nine, bogeying both the par 3s before a birdie on the long 17th.