By Monitor l YESTERDAY afternoon's astonishing match between Tim Henman and the talented but unseeded Finn, Jakko Nieminen, who had never played on grass before, showed that we are all going to need steady nerves in the coming days. Two sets down in fast order, Henman looked destined for an early exit from the championship but somehow pulled the game round until after more than three hours play he eventually took the final set. These cliff-edge encounters are not good for those of us of advanced years and the knowledge that Henman will probably lose in a semi-final after shredding our nerves in several preliminary matches is not a prospect to savour. On the other hand, what other sporting spectacle offers such tantalising shifts of fortune as does tennis. Whoever devised its scoring system was a genius because it can sway the fortunes of a game in a matter of a few points. Then there is the gladatorial aspect of a game played on one of the sport's great courts, of which Wimbledon Centre Court is surely the greatest. There have recently been some silly suggestions from people who shoujld know better that the day is not far off when Wimbledon's grass will be concreted over in order to provide a surface that all the top players are familiar with. The oft-quoted level playing field is what they seem to have in mind. But surely lawn tennis is infinitely to be preferred to the clay and concrete and wood of the articifical surfaces? If there is to be change it should be in the direction of finding ways of growing grass courts in even the most inhospitable climates. Meanwhile, those on Henman Hill in London SW 19 and those in Henman Hell in front of our television sets can only wish Tim Henman the very best of luck and more amazing improbable escapes from certain defeat until he finally holds aloft the cup we all want to cheer.