Thomas holed an 8-foot putt on the par-5 18th for a 64 to set the 36-hole scoring record at 123.?
He also had a five-shot lead over Gary Woodland going into the weekend, when the focus shifts from records to a trophy. Coming off a 59, Thomas started slowly until running off four straight birdies around the turn.
He bogeyed the 17th hole from the bunker, then hit out of the rough to 8 feet on the last hole to atone for it. Woodland had a second straight 64 and was at 12-under 128.
Webb Simpson (65) and Charles Howell III (66) were among those at 9-under 131, while the group at 132 included former Navy lieutenant Billy Hurley III (68) and Miguel Tabuena of the Philippines, who had his best year in 2016 on the Asian Tour.
Thomas' first round on Thursday was the eighth sub-60 round in PGA Tour history with a 15-foot eagle putt on the final hole. The only other player to shoot 59 in the first round was Paul Goydos in the 2010 John Deere Classic. He finished second that week to Steve Stricker, who opened with a 60.
The Associated Press contributed to this report.?