At the beginning of maximal exercise your oxygen supply cannot meet your energy demands, and therefore work is anaerobic. During this period you are using your phosphocreatine (PCr) stores to supply immediate energy using the ATP-PCr system (also called the immediate, or phosphagen, energy system).
A muscle cell has a small amount of ATP floating around that it can use immediately, but only enough to last for about three seconds. However, ATP can be resynthesized from a compound called creatine phosphate until these stores are depleted.
The PCr system can supply the energy needs of working muscles at a high rate for only 8-10 seconds. In tennis, the ball is in play for about 10-30% of the time and out of play for about 70-90% of the time (i.e. 3-8 seconds of work with 15-25 seconds of rest per point). These data suggest that the ATP-PCr system is mainly used during tennis, with restoration of the PCr in the recovery periods between points and games.