Wondering how Kevin Federline and Britney Spears have kept their relationship alive? Chalk it up to patience — on his end. "She thought she could play me," he explains of their early days in GQ's September issue (on newsstands August 22). "She was playing games, testing me to see where my head is at, see if she could get away with this or that." Federline said he stuck it out through the relationship's rocky start and proved to his future wife that he was interested in the woman, not the fame. The dancer-turned-rapper also revealed to GQ that he toilet-papered 98 Degrees' tour bus, aced the GED — "Not that it's the hardest thing in the world" — and has a collaboration with Britney recorded and ready to roll. "After she has the baby and gets skinny, we're gonna hit 'em with this sh--," he said.