MY mates and I never had much time for maths.

If it hadn't been for Dairylea we would not have known what a triangle was, we thought rhomboids were what caused our dads such woe at ablutions and we believed symmetry was the study of vests.

My mate, Tam, also had hidden shallows. His genius was that he could take a little ignorance and spread it into a variety of subjects. For example, he would never read a novel because it was not "real". Yet he could watch cartoons, go to the pictures or entertain hopes of losing his virginity without questioning the reality of any of it.

He brought a book to me last week with the sort of ambling pride that a cat has when it dumps a dead bird on one's doorstep. It was by David Foster Wallace, a succession of names Tam would have previously believed was a Third Lanark half-back line of the 1950s. "It is brilliant. It is about maths and tennis," he said. I could not have been more surprised if he had said he had enjoyed a two-hour chat with his missus on the meaning of relationships and then washed behind his ears.

The truth is, of course, that The David Foster Wallace Reader - the book, not Tam - is about so much more but I did not argue because a) it is as brilliant as a new pair of white trainers; b) Tam should never be discouraged from reading anything that is not on the back of a sauce bottle; and c) he had provided me with the subject for this week's witterings.

Wallace, indeed, wrote about tennis. His most famous essay just might be Federer Both Flesh and Not, though I prefer Derivative Sport in Tornado Alley. Both, however, enhance a trip this week to watch the best tennis players in the world, minus Rafa Nadal, contest the Barclays ATP Tour finals in London.

Tornado Alley reinforces Wallace's right to write about tennis. He was a "near-great junior tennis player". In contrast, my interaction with the sport was trying to winch lassies round the back of Busby Tennis Club.

Wallace's experience of tennis is brilliantly revealed in the Federer essay but its depth owes more to the writer's unfailing power of insight and, of course, his subject's greatness. Now, Wallace can be wrong on Federer. He writes of the Greatest Of All Time coming out on to court wearing "a buttermilk sports coat", insisting "it doesn't look absurd with shorts and sneakers". It does. Federer looks like a lounge singer on a cruise who is waiting for his trousers to be ironed.

But Wallace evokes Federer moments on court with insight and with an unbridled joy. How about this? "Federer's forehand is a great liquid whip." This is the gem that adorns just one of Wallace's sentences and it invokes an extraordinary poignancy.

At 33, Federer heads to the O2 with a genuine chance of winning yet another tour finals. His greatness is still visible but it is not resilient. His lithe perfection, his deceptive strength helped create modern tennis as the power game that makes huge demands on the body, particularly in grand slams where seven matches must be won in a fortnight.

Federer once seemed to stroll up this mountain of physical strain, winning 17 majors. But no more. His last grand slam title was Wimbledon 2012. To Federer, this must seem like prehistoric times.

He knows Rafa is too good on clay. He suspects Novak Djokovic now has his measure, particularly at the end of a grand slam fortnight. He shares an 11-11 head to head with Andy Murray. He has also been blasted out of grand slams by younger, more powerful players who sense a weakness in the once-invincible champion.

Yet he plays on. He lives not in hope but in belief. Federer has faith in himself. It is what has sustained him in moments of crisis. It is what bolsters him when that question arises of just why is he still chasing a yellow ball in a glorified gym hall.

He may not be a grand slam favourite. He is still a genius. This statement of fact will lead me into arenas whenever he is playing. He must be savoured because human beauty, corporeal brilliance, is transient, even fragile. David Foster Wallace, chronicler of Federer and so much more, committed suicide six years ago. He was 46.

The David Foster Wallace Reader is published by Hamish Hamilton at £25