It would seem that the most likely factors here are
blood sugar (&
glycogen) levels and
conditioning. If the OP is depleting glycogen stores or is experiencing low blood sugar levels, this could account for fatigue or "bonking" (aka "hitting the wall"). If
inadequate carbs are ingested, glycoen levels could be very low (which could result in low blood sugar levels). Another possibility is consuming a high
glycemic (
load) snack before competition. This could result in a blood sugar spike. If insulin overcompensates for this spike, depressed blood sugar levels could result.
Conditioning is another possibilty. Altho' people often believe that tennis is not a aerobic activity, it should be noted that the
aerobic system is employed for tennis -- a good
aerobic (fitness)
base is a must for sustained tennis acitivity. There are also 2 anaerobic systems that are employed when playing tennis -- an
immediate system (ATP-CP) and a
short-term system (Anaerobic glycolysis).
Interval trining can be used to improve anaerobis systems. For more info on these aerobic systems & anearobic systems in tennis, take a look at the following posts:
tt.tennis-warehouse.com/showthread.php?p=2185965
tt.tennis-warehouse.com/showthread.php?p=2192308
Should s/he stretch before or after the cardio workout? Or not at all.
Don't believe this has any bearing on energy levels.