Det här låter som den andra boken från serien Tripods :
The City of Gold and Lead (1968)
After a year in the White Mountains, the resistance charges Will, Beanpole, and a German boy, Fritz, to infiltrate a Tripod city by competing in a regional sporting exhibition. Will, a boxer, and Fritz, a runner, win their respective contests, while Beanpole fails to win in the jumping events.
The winners are taken to the Tripod city in a pressurised dome astride a river. Inside the city, the boys discover the Tripods' operators, whom they refer to as the 'Masters'. [...]
(source: Wikipedia)