Killeen, the overnight leader by five, took a step back Saturday with a 1-over 73 in the third round, but still held a comfortable cushion heading into Sunday.
The 26-year-old Texan bogeyed four of his last six holes to finish at 9-under 207.
Tom Gillis shot a 69 and moved up from a tie for seventh place into a share of second at 5-under 211. He was joined there by Joe Daley and Aron Price, who both posted 72.
James Love (73) was three shots further back at 2-under 214.
Only seven more players were within nine shots of Killeen, who was as low as 13-under par Saturday after collecting three birdies in a row from the eighth hole.
'I had it going in the middle and hit some good shots,' said Killeen.
Perhaps finally realizing where he was, Killeen, who matched the course record with a 65 in the first round, went on to bogey four holes from the 13th, letting the field back in a little.
Like the rest of the players, he dealt with windy conditions for another day.
'I played pretty well on a difficult day. That was a tough finish, but I'm not disappointed with any of it,' said Killeen. 'I kept trying to hit good shots, but conditions were so tough that par was a good score on any hole.'
The scoring average dropped from 75.032 in the second round to 74.770 on Saturday, but only seven players among the 61 remaining were able to break par.
'You can't trick it around on a day like today,' said Gillis. 'In order to catch somebody that's out there that far you've got to have a day like today, you've got to play solid.'
Added Killeen: 'You just have to play the course. I wasn't really aggressive, but you can't be too conservative because you've got to hit a lot of long irons.'
Sunday, Killeen will be looking to become the third first-time winner this season.
'I'm not going to change anything I'm doing,' he said. 'I'm definitely not nervous when I'm out there playing. The nervous part is in between when your mind starts thinking about everything.'