Mental toughness?
Why do the very best players win when they rarely lead the stats in aces etc?
Mental toughness is the answer. For instance, there was a time when Federer lost a lot of matches to lesser players, but in 2002 Roger made the decision that he needed to be mentally tougher in his matches and hang in there longer, which made all the difference – and he became the premier player for many years, until Nadal built up the belief that he could win all all surfaces, not just clay.
Nadal won his grand slams from sheer dogged determination and mental strength, mixed with his fabulous relentless groundstrokes, and won against Federer when no-one else could – by simply refusing to mental fold the way most players have done when they walked on court against Federer between 2002 till 2008.