Ryan Palmer, a West Texas native who cut through winds as strong as 25 miles per hour (40kph), had the round of the day with four-under 68.
Round of the day: Koepka started the day four shots off the pace, but he made a quick move up the standings with five birdies over his first eight holes.
Amongst the group in a share of third place is Australia's Cameron Smith who actually took the lead through the opening nine holes before dropping three shots in four holes as he headed into the back nine.
Chappell, a six-time PGA runner-up who was seventh at the Masters two weeks ago, put approaches at the par-5 14th and par-4 17th inches from the cup to set up birdies and sank a 12-foot birdie putt at the par-3 16th to seize the lead.
"I just didn't give up any shots out there", Grace said.
"You appreciate it every time you win because it's so hard", Koepka said of Chappell's win. His day was relatively uneventful until the 14th when he got his first birdie of the round.
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"I don't know what the weather has got in store for us tomorrow, but if it's a good day it's going to probably take a couple under (to win)", Grace said.
NOTES: Since 2000, seven of the 17 second-round leaders/co-leaders of the Valero Texas Open have gone on to win, including two recent past champions, Steven Bowditch (2014) and Jimmy Walker (2015). He made me had to do it the right way.
John Huh, who was paired with Chappell for the final round, struggled with a five-over 77 to finish in a five-way tie for 22nd place at two under. He had three bogeys, a double-bogey and three birdies on the outward nine, which led to a 2-over 38. He also missed an eight-foot birdie putt on the drivable par-4 17th.
Carlos Franco and Vijay Singh teamed to shoot a course-record 12 under to rally from seven shots behind and win the PGA Tour Champions' Bass Pro Shops Legends of Golf at Big Cedar Lodge in Ridgedale, Mo.
Laird looked confident when he birdied the second, but he bogeyed three of the next four holes and laboured to a three-over 75 to close on three under par.
Camilo Villegas emerged as a strong contender when he birdied five of the first six holes, but the Colombian stumbled after the turn and dropped three shots in two holes before completing a disappointing 70 which left him six off the lead.