India, Oct. 8 -- Novak Djokovic has just reminded us why longevity in professional tennis is so extraordinary. The 39-year-old Serbian completed a perfect return to Beijing by winning his seventh China Open title, extending his record at the tournament to 34-0 and collecting the 102nd ATP title of his career. Only Roger Federer, with 103, and Jimmy Connors, with 109, have more in the Open Era. Djokovic also became the first player to win at least one tour-level title in 21 consecutive seasons.

But the numbers only tell one part of the story. Speaking after his Beijing triumph, Djokovic offered a fascinating explanation for how he has survived nearly two decades at the top - and why he considers tennis a particularly "cruel" sport. His co...