FORT WORTH, Texas -- Webb Simpson felt almost as if he had just played a round in a major after a steamy Saturday at Colonial.
For the 2012 U.S. Open champion, and pretty much everyone else at Hogan's Alley, it was a mentally draining day at the traditional tree-lined course where temperatures were in the upper 90s, with a heat index of 108 degrees and wind gusting to 25 mph.
"There is so much thinking involved on every shot,'' Simpson said.
Even so, Simpson shot a 3-under 67 to break out of a crowd and take the third-round lead.
"It was a good grind day,'' Lee said. "It's tough out there with the wind blowing sideways.''
Defending champion Jordan Spieth shot a 68 with a caddie switch after making the turn, and was at 4 under.
Part of a four-way tie for the 36-hole lead, Simpson went ahead alone to stay after consecutive birdies to start his back nine. He made a 6-foot birdie putt at the 387-yard 10th and a 12-footer at the 630-yard 11th before finishing with seven consecutive pars.
It is only the second time in 12 years at Colonial that the 54-hole leader has more than a one-stroke advantage.
Simpson leads after three rounds for the fifth time in his career, and first since the Shriners Hospitals for Children Open in October 2013 that was also the last of his four PGA Tour victories. He lost in a playoff to Hideki Matsuyama at Phoenix in February.
"Definitely feel like (the season has) ebbed and flowed,'' he said. "I feel like that's starting to happen more, where I'm part of the conversation of contending. That's a good feeling. I know what I'm doing is working.''
Playing in the final group, Simpson birdied the first two holes Saturday. He gave one of those back with a bogey at the par-3, 192-yard eighth after missing the green with his tee shot. His only other birdies were at Nos. 10 and 11.
Spieth said it was "a no-brainer situation'' to make a caddie switch because Greller felt so bad. Greller told Spieth that he'd be ready for Sunday's final round.
Damon Goddard, the personal trainer for both the player and the caddie, carried the bag the last seven holes. Spieth had a bogey at the 14th and two birdies down the stretch.
'He's caddied for me before. Michael was really sick in Houston in 2014. Damon caddied the first round. We went bogey-free, 2 under,'' Spieth said. "I let Damon down today. I made my first bogey with him on the bag by three-putting.''
Lee and Kisner were also tied for the second-round lead, along with Scott Piercy who shot a 72 to fall five strokes back.
After three birdies and three bogeys his first eight holes Saturday, Lee had a steady stream of pars until finishing with a 16-foot birdie.
"I really needed that to have a good momentum starting tomorrow,'' Lee said. "It was a good grind out there. It's not easy out there when wind is blowing sideways every hole. It's very hard for me to commit every shot.''
Casey made a nearly 25-foot birdie putt that had just enough speed to fall into the cup at No. 18. He was 1 over for the day without a birdie until starting his back nine with three birdies in a four-hole stretch that included a bogey.
"I had missed a lot of good looks for birdies and opportunities, although I'm not complaining,'' Casey said. "Happy I shot under par.''
Piercy was still within a stroke of the lead at 8 under after a par-saving 13-foot putt at the 12th hole after driving into the rough and hitting his approach in a greenside bunker there. But he promptly hit his tee shot at the par-3 13th into the pond on way to a double-bogey, and couldn't avoid bogeys after wayward drives and going into greenside bunkers at No. 15 and 18.