Quads Lagging

I have found the best is doing 2sets of heavy squats in 4-6 rep range. Then finishing off with widowmaker. However I can only get to about 14-15 reps before i have to rest just because i can feel my form getting bad because of fatigue and my lower back will start to hurt due to losing the arch I can’t help it, its just from fatigue so i rest for 1 min and finish off the last 6-5 reps.