The big news in the paddock after the 1993 season was the retirement of defending World Champion Alain Prost after winning his fourth title, and his replacement at Williams by long-term rival Ayrton Senna. It was commonly thought that Senna's move to Williams was a factor behind Prost's retirement, which in a later interview was proved to be correct. Prost's retirement also saw the second season in succession where the reigning World Champion would not defend their title after champion Nigel Mansell had spent 1993 racing in (and winning) the US based Champ Car series.