As you see in the picture above, there are four phases: initial phase (before training), training phase, recovery phase and supercompensation phase.
In the initial phase, the athlete has a base fitness level. Once the training start, the athlete begins to introduce training stimulus that are much higher than what the body is used to. This training bring fatigue which is reflected in declining the performance.
In the recovery phase, the athlete has no choice after the decline in the performance to introduce recovery. During recovery, the performance starts to return to original baseline fitness.