Most blisters are not a mystery. Your foot is moving inside the shoe. The sock is wet. The upper is a floppy bag. Two hours of clay later, the little toe is chewed.
Lock the cut
Tennis is a lateral sport. You plant, you cut, you recover. If the upper doesn't hold, the foot rolls inside the shoe. That's how you roll an ankle. That's how you get hot spots.
The SUPACRITICAL upper is breathable mesh that actually locks your foot in place when you slide and cut. Not a sock. Not a brick. Hold, then breathe.
Wet socks, long matches
The insole has to stay comfortable when you're sweating through a long match. If it packs down or soaks through and goes slimy, the match is over even if the score isn't.
The test
If a prototype produces blisters, it does not ship. Players should forget the shoe. That's the point.