Leishman finished 54 holes at 17-under 199 and is five ahead at Midland Country Club.
Michael Boyd managed a four-under 68 on Saturday and is alone in second place at minus-12.
Cameron Percy (66) and Brendon de Jonge (67) are knotted in third place at 10- under 206.
Leishman flew out of the gate on Saturday with four birdies in his first six holes, including three in a row from the second. He added birdies at nine and 12 and found himself four clear.
Things tightened considerably at the 15th.
Boyd recorded his sixth birdie of the round at the par-3 16th. That got Boyd to 14-under par, and he ended up getting within two after a Leishman bogey at the par-5 15th.
With the advantage down to two, things took another turn.
Boyd double-bogeyed the 17th hole after finding the water and Leishman birdied No. 16 to get a five-stroke lead. Both players parred the 18th to put Leishman firmly in the driver's seat with one round to go. Leishman has never won on the Nationwide Tour.
'I was a little more conservative off the tee today and it was another good day with the putter,' acknowledged Leishman. 'Hopefully I can do the same tomorrow.'
Aron Price shot a four-under 68 and is alone in fifth at minus-9.
Ricky Barnes (71) and Spencer Levin (72) are knotted in sixth place at 8- under 208.
Justin Hicks (68), Won Joon Lee (71), Bob Burns (70) and Brendon Todd (73) share eighth at 7-under 209.