There are a number of factors which turn an exciting rivalry into an epic, enduring duel that transcends the sport.
The core talent. The blend of personalities. The gripping encounters on the biggest stages.
The tussle between Italy's Jannik Sinner and Spain's Carlos Alcaraz - ranked one and two in the men's game - has all those components.
It also has arguably the most important ingredient: each player being pushed to a greater height by the other.
On Sunday, the pair will meet again in the Wimbledon final - where Sinner aims to win his first title and Alcaraz bids for a rare third in a row.