Daniel Chambliss in The Mundanity of Excellence, which I came across through James Clear’s newsletter:

At the higher levels of competitive swimming, something like an inversion of attitude takes place. The very features of the sport that the ‘C’ swimmer finds unpleasant, the top level swimmer enjoys. What others see as boring—swimming back and forth over a black line for two hours, say—they find peaceful, even meditative, often challenging, or therapeutic. They enjoy hard practices, look forward to difficult competitions, try to set difficult goals. Coming into the 5:30 A.M. practices at Mission Viejo, many of the swimmers were lively, laughing, talking, enjoying themselves, perhaps appreciating the fact that most people would positively hate doing it. It is incorrect to believe that top athletes suffer great sacrifices to achieve their goals. Often, they don’t see what they do as sacrificial at all. They like it.

Alongside attitude, Chambliss identifies discipline and technique as key factors of excellence.