Simpson leads Casey, Lee by two

-Simpson cards three-under 67

-Casey, Lee both birdie final hole

-Spieth, Garcia five behind

Webb Simpson lipped-out a birdie chance at the final hole and had to settle for a two-stroke lead over Paul Casey and Danny Lee after the third round of the Dean & DeLuca Invitational in Fort Worth, Texas on Saturday.

2012 U.S. Open champion Simpson's 15-foot putt was headed for the centre of the hole but broke sharply at the end, caught the low side of the cup and spun out.

But that was about the only lament the American had after carding 67 to post a nine-under-par 201 total on another breezy day at Colonial Country Club.

"I'm happy with the day," Simpson, a four-times PGA Tour winner, told PGATour.com. "I kept the ball in the fairway, putted well, putted consistent. I'm trying to shoot under-par every day and today was another good one."

Englishman Casey (68) and South Korean-born New Zealander Lee (69) were the closest pursuers on seven-under after finding the final green less frustrating, both making birdie.

Americans Stewart Cink (66) and Kevin Kisner (70) were three off the pace, while defending champion Jordan Spieth (68) and Masters champion Sergio Garcia (71) were five behind.

Spieth played the final seven-plus holes with a stand-in caddie after his regular looper Michael Greller fell ill in the oppressive heat and humidity. Trainer Damon Goddard carried the bag the rest of the way.

"Michael went down with heat exhaustion," said Spieth. "When we were walking down the 11th he said 'this is my last hole.' I thought he was acting a little strange, a little quiet."

Casey, who like Lee has just the one PGA Tour win, has seen his putting touch improve recently.

"Ball-striking has always been my strength," said Casey, who also has 13 European Tour wins. "Weeks I've putted well I've always had a chance to win, or won."

"I like Texas. My only other (PGA Tour) victory was at Houston. We can't predict the future but I'm very excited with how I've been playing."