McCarron, the Charles Schwab Cup points leader and a three-time winner this year, birdied the par-5 16th and par-3 17th in a bogey-free round at rain-softened Warwick Hills, the longtime home of the PGA Tour's defunct Buick Open.
"Conditions were a little bit soft out there from the rain that we had and there were some gusty winds out there," McCarron said. "It was difficult to kind of judge distance with some of our iron shots, so it was hard to get it close. I watched Retief Goosen play a spectacular round. He hit the ball [within] about 10 feet almost every hole, at least for the first 13, 14 holes. It was very impressive. I kind of just rode his wake in there and was able to make a couple birdies coming in."
Goosen, the Senior Players Championship winner for his first victory on the 50-and-over tour, bogeyed the 17th.
"The first five, six holes played into a strong wind this morning and I really hit some really good shots in those first few holes and made a few birdies," Goosen said. "I was 4 under through six, so I was off to a great start in tough conditions. After that I had my chances to go really low, missed a few putts, short ones. Then, obviously, very much a mental mistake on No. 17. Hit a bad shot there and caught a poor lie in the bunker and didn't get it up and down. So, a bit of a disappointing finish."
Gillis finished his bogey-free round with a birdie on the par-4 ninth.
"What we have is terribly soft," Gillis said. "We had a lot of rain the last two days. We had about a 15 mph wind that made some challenge on some shots, but overall being soft, you could get to the pins. It was accessible."
"I kept the ball in the fairway," the 65-year-old Haas said. "You have to keep the ball out of the rough here because it's really juicy and wet and the ball's just not going anywhere, so I did that most of the day."
Bernhard Langer topped the group at 68.
Fred Couples opened with a 70. He won the 1994 Buick Open.
Defending champion Paul Broadhurst shot 71.
Two-time major winner Angel Cabrera had a 72 in his tour debut.
Three-time Buick Open winner Vijay Singh shot a 76, playing alongside McCarron and Goosen.