The slide is a skill and the shoe is only half of it

Outsole grip is sold as an unambiguous good, and at the end of a long rally it is what wrecks a knee.

Grip is not a virtue. It is a variable you want the right amount of, and the tennis shoe market is built on selling more of it than most players should want.

Consider what happens at the end of a wide rally ball. The player arrives at speed and has to stop. On clay they slide, which spreads the deceleration over a metre or more of travel and lets the body absorb the change gradually. On a hard court with a high-grip outsole the foot stops immediately and the deceleration is taken by the ankle, the knee and the hip in a fraction of a second. Same speed, same body, radically different loading. Repeat it four hundred times in a match.

That is why the professional habit of sliding on hard courts, which looks like showing off, is closer to self-preservation. It is also a genuine skill, and it is not simply doing on concrete what you do on clay. The controlled hard court slide requires the outside foot to arrive at a shallower angle with the body already lowered, so the shoe scrubs rather than catches. Get the angle wrong and the shoe grips halfway through, which is the moment ankles go.

The endurance argument runs alongside. A player deep in a third set is not just tired in the lungs. The stabilising muscles that manage those decelerations fatigue first, so the same movement that was safe in the first set is being performed with less control. Fatigue is a mechanical risk, not only a performance one, and rally endurance training that only builds cardiovascular capacity is missing the part that actually breaks.

My argument about equipment is that players choose an outsole for the surface and almost never for their own movement pattern. A player who slides well wants less bite and pays for it with a slightly slower change of direction. A player who plants and pushes wants more. Buying whatever the current champion wears is how you end up in a shoe designed around somebody else's footwork.

There is also the court to consider, and it varies more than the tour admits. Two hard courts at different events can have very different friction, and players discover this in the first practice by sliding and finding out whether the court permits it.

Nobody puts a friction number on a court. They should.

You can feel the difference in ten minutes and it changes how you move for a fortnight.