First of all, heat is not a friend of your tennis or racquetball racket. A racket left in a hot car or garage will be affected by the heat in a few possible ways.
One of the ways is your strings. Heat will cause them to lose tension faster than normal. This will be felt in a loss of power, added vibration and for tennis players, less spin potential. While they may not break, the reduced performance could be noticeable enough to have you losing more points.
Another problem heat can cause is a premature breakdown of the grip. Particularly, rackets left in a hot garage for a long period of time will often have grips that look as though they have "melted" slightly. This will reduce how well you can grip the racket securely.
The worst thing that can happen to a racket left in a hot car is damage to the frame. Excessive heat can break down the epoxy holding the graphite together, shortening the life of the racket, and in the worst case, racket breakage.
If you must keep rackets in your car, you may want to invest in an insulated racket bag which will protect the rackets from much of the heat. Obviously, the best option is to keep them out of hot cars or garages during the summer months, helping them last much longer.
