Nadal had the toughest opposition

The argument

From the emergence of Roger Federer as the world's best tennis player in 2004 up to the early 2020s, men's tennis has been dominated largely by the "Big Three" players: Federer, Rafael Nadal, and Novak Djokovic. While each of the three has had unprecedented success and has had to contend with the other two on numerous occasions, Nadal has faced a relatively steeper challenge from the Big Three era than either of his rivals have. Federer, as the oldest of the three and first to achieve breakout success, was able to accumulate a large portion of his Grand Slam titles in a relatively weaker era of competition. Federer won 12 Grand Slam championships from 2003-2007, dominating less accomplished players while Nadal and Djokovic were still developing. Djokovic, as the youngest and last to come into his prime years, similarly benefited later in the Big Three era. Thus Nadal, in between the ages of Federer and Djokovic, had his athletic prime years overlap on either end with the prime years of either Federer, or Djokovic, or both. This forced Nadal to win his biggest championships under the most difficult possible circumstances: in direct competition with not one but two other legitimate candidates to be considered the best player of all time. Yet in spite of this fierce competition, Nadal succeeded, and compiled a list of victories comparable to those of his rivals and any other player in history.

Premises

Counter-arguments

Nadal came to prominence after Federer, but before Djokovic reached his prime. Nadal won 3 of his 19 grand slams before Novak won his first, and 9 of his 19 before Djokovic's dominant period began in 2011. Djokovic had the most challenging entry into top-level tennis competition, facing Federer and Nadal in their prime.

Rejecting the premises

[Rejecting P1] This premise is common ground and does nothing for the position; every argument in this question starts from the same era. [Rejecting P2] The characterisation of the early 2000s as a weak field is asserted, and the counter-argument supplies the correction: Nadal won several major titles before Djokovic's first and roughly half of them before Djokovic's dominant run began. The premise also proves too much, since the same reasoning would discount Nadal's own early titles as won before his rivals arrived. [Rejecting P3] Overlapping with both rivals at their peaks is genuinely a hard road, but no measure of it is offered — not head-to-head records, not titles won against top-ranked opponents, not the quality of beaten finalists. Strength of opposition is also one criterion among several in a greatest-of-all-time comparison, and the argument never explains why it should outweigh totals, longevity or breadth across surfaces.