E.C. Tubb 
Galactiad 
Cap Kennedy Book 17

The things from beyond the Milky Way galaxy found the intelligent races of our universe amusingly slight. To them, possessors of vast cosmic power, the strivings of various humanoids to outdo each other were a source of contemptuous entertainment.They established a contest between the worlds. It would be an Olympiad of the whole galaxy — a Galactiad. Let these puny interstellar intelligences meet each other in contest. Pit one against the other — and let the losers beware!Earth had its team — a mixed group of powerful athletes and genius scientists. Because other worlds did not always believe in the ideal of good sportsmanship, they had to confront the reality. Win at all costs . . . or goodbye to humanity!