When Hamilton joined Mercedes in 2013, he was paired alongside old karting teammate and friend Nico Rosberg. Over their four seasons as teammates, a period of Mercedes dominance Formula One, the pair's relationship became strained and, at times, led to volatile confrontations on and off the track. Hamilton and Rosberg were first teammates in 2000, when they were in karting. They raced for Mercedes Benz McLaren in Formula A, where Hamilton became European champion, with Rosberg not far behind. Robert Kubica, who raced with them before Formula One, recalled how they were competitive both on and off the track, saying "they would even have races to eat pizza, always eating two at a time". Sports journalist Paul Weaver contrasts their upbringings: Rosberg, an only child, was born in Germany but brought up in Monaco and was the son of a wealthy former Formula One world champion, Keke Rosberg, whereas Hamilton was born on a council estate in Stevenage, and his father had to work multiple jobs to fund his son's junior racing.