Lewis sank eight birdies, including four in a row from the 15th hole, offset by a bogey on the par-three fifth while Schwab had six birdies and an eagle on the fourth at the Montgomerie Maxx Royal golf course.
"I wasn't comfortably rolling it on the front nine even though I hit some good shots," Lewis said. "On the back nine I continued to hit good shots and I felt more confident on putts.
"That's going to be the difference this week between the guy that wins and the guys that don't."
American David Lipsky, Belgian Thomas Pieters and Sweden's Alex Noren were tied for second at six-under par while six others -- including defending champion Justin Rose of England -- were a shot further back.
Rose, who eagled the par-five 11th with a delicate chip on to the green, is looking to win his third title in a row at the Rolex Series event after winning the trophy by a single stroke in 2017 and in a playoff last year.
Race to Dubai leader Bernd Wiesberger signed for a one-under par 71 while The Open champion Shane Lowry finished with a par score after sinking only one birdie.