Earth Force I
by Robert C. Adams
Jason and Kai, two friends from college, are both competitors in the World Championships for Earth Force, a mech-fighter game created by billionaire tycoon, Howard Hunt. Hunt has secretly been building real battlesuits, based on the technology of a warlike alien race, the Valorians.
When the Valorians show up on Earth, Hunt recruits the top Earth Force gamers to defend the planet.