The likes of McEnroe and Connors were big personalities that transcended tennis. There have been better tennis players than those 2 but most people who don't even watch tennis know who they are.
it depends on whether you define 'legend' as the overall persona of a player or their overall achievements. both can be very different, for instance kournikova is a legend of womens tennis if we talk about transcending the sport, but obviously not one on court.
i always feel mcenroe was hugely helped to reach 'legend' status by his persona where his results lacked.