The 53-year-old Schuller could earn his first PGA Championship berth in 13 years as he enters the final round of the PGA Professional Championship well within the top-20 cut.
The 47-year-old longtime PGA Tour veteran posted his second straight round of 68 or better to move in front at the halfway point at Turning Stone Resort.
Mark Brown shot a 4-under 68 to trail co-leaders Ryan Helminen and Craig Bowden by one.
Brooke Henderson thwarted Lydia Ko's attempt to become the youngest player to win three consecutive majors with her win at Sahalee Country Club.
Scotland's Catriona Matthew enters Sunday's final round at Sahalee Country Club in Sammamish, Wash., three shots off leader Lydia Ko.
Rocco Mediate posted a 5-under 66 to earn a three-shot victory of Colin Montgomerie.
Monty is trying to become only the ninth player since 1900, at any level of professional golf, to win the same major championship three times in a row.
Rocco Mediate shot 5-under 66 Friday at the Senior PGA Championship to move to 14-under-par 128, which bests the 36-hole tournament record by four shots.
Out of nowhere, Rocco Mediate opens the Senior PGA Championship at Harbor Shores by one thanks to an opening 62.