Kuchar and English opened with an eagle on No. 1 and followed with three straight birdies. They had two birdies from Nos. 5-9 and three straight birdies from Nos. 13-15 on the back nine. They finished with an eagle on No. 17 and birdie on No. 18.
"Having Kooch as a backup guy is pretty good," English said. "I know if I mess up or I miss a putt, I wouldn't want anybody else hitting a shot or putting behind me."
"Having a partner lets you be a little bit more aggressive, lets you be a little more free-swinging," Kuchar said. "We've just formed a good partnership and get in a pretty good groove out there."
Since being paired in 2013, Kuchar and English have gone 79 under in their seven rounds. That's an average of a little better than 11 under per round.
Horschel-Mahan birdied their first three holes and went birdie-eagle-birdie to end their day. They also eagled No. 14.
The format Friday will be modified alternate shot. Horschel said they'd be pleased to go 13 under again.
"I think we're going to come in here with some pretty big smiles on our face," he said.