McCall, pictured, quit Somerset Park in September to return to his first love but he was ruing what he left behind as a double from Craig Moore and goals from Michael Moffat and Alan Forrest fired Ayr to a runaway victory.
Kerr, who played under McCall at Dundee United, Falkirk and Ayr, showed he had learned more than a thing or two from his mentor as his team stayed hot on the heels of leaders Dundee United.
McCall, left with much to ponder, said: “If you put aside the sideshow, Ayr were very good and we were very poor. The second half was far better but that’s only because Ayr took their foot off the gas.
“Everybody talks about top four but we’re not. We are in a relegation fight and although there will be three or four other teams dragged into it, we need to start winning. Today wasn’t good enough and I take full responsibility.”
Ayr almost caught the visitors cold when Moore forced a good save from Scott Fox only for Luke McCowan to blast the rebound over. Kerr’s men deservedly took the lead when Jamie Houston fired in a low cross which Moore swept into the bottom corner.
It was soon 2-0 as Forrest slipped the ball to McCowan whose cross was met by Moffat, stooping to head home his 96th goal as an Honest Man.
McCall, who had started the game in the directors’ box, immediately scurried down to the trackside in an attempt to arrest his team’s dismal start.
He hooked Shea Gordon and sent on Lewis Mansell after 28 minutes but still Ayr dictated the play as Thistle struggled to get a foothold.
Forrest went close to a third goal just before the interval when he fired narrowly over from McCowan’s perfect pass.
Rampant Ayr buried Thistle with two goals in three minutes at the start of the second half, Moore fizzing home a 25-yard rocket before Forrest capped a mazy run with a curling finish into the bottom corner.
It took Thistle 55 minutes to register their first serious effort on goal but Joe Cardle’s shot from distance smacked off the outside of the post.
Cardle did pull back a consolation goal when he rose unchallenged to head home a Ryan Williamson cross.
Ayr chief Kerr, who made it two victories from three games in charge, claimed: “We were outstanding from start to finish. We have a togetherness and it’s been smooth since I took over.”