Bateman, who carded a 6-under 66, and Brehaut are tied at 17-under-par 343 after five rounds.
Alex Cejka (64) and overnight leader John Morgan (71) are tied for third place at 16-under-par 344. Chris Anderson is alone in fifth at minus 15.
The top 35 finishers and ties after Monday's sixth and final round will receive their PGA Tour cards for the 2003 season and play has been contested at PGA West's Nicklaus Tournament Course and the Stadium Course.
Brehaut played Sunday's round at the Nicklaus Tournament Course and opened on the back nine with three birdies in his first six holes. It was on the back nine where Brehaut did his damage with six birdies over the nine-hole span to reach first place.
'That was probably my two best rounds of the year right there,' said Brehaut. 'I got off to a good start and then my putter was just smoking hot on the back side. I made everything and that is why I shot 63.'
Bateman collected three birdies and a bogey on his front side, also the back nine at the Nicklaus Tournament Course. He added four birdies in five holes on his second nine but did not secure his spot at the top of the leaderboard until he holed a 12-foot par save at his last hole.
'I played well today,' said Bateman. 'My whole game plan all week has been to try and keep it in play and hit a lot of fairways and greens. I made a few putts today and that always helps at Q-School.'
Joel Kribel (67), Scott Laycock (72) and Anthony Painter (71) share sixth place at 14-under-par 346.
Donnie Hammond and James McLean shot matching rounds of 2-under-par 70 and are tied for ninth at minus 13.
Martin made one of the biggest moves in Sunday's fifth round. He shot a 5-under 67 and jumped 16 spots into a tie for 21st after he came into the round in a share of 37th.
'There is still a lot of golf to play,' said Martin. 'Hopefully I'll just hit some good shots and keep myself calm tomorrow.'
More from the PGA Tour Qualifying Tournament: