The rivalry between Roger Federer and Rafael Nadal is the "greatest in the history of tennis," according to journalist Sebastian Fest.
On Thursday, Federer called time on a glittering career that brought 20 grand slam men's singles titles – a tally only bettered by Nadal (22) and Novak Djokovic (21) – with his swansong coming at next week's Laver Cup.
Many of the sport's biggest names paid tribute to the Swiss great, including the Spaniard, who said it had been "an honour and privilege to share all these years with you."
Those years brought 40 meetings in grand slam competition, including a record nine finals, with their 2008 Wimbledon championship showdown widely regarded as the greatest match in tennis history.
Speaking to Stats Perform, Fest – author of Roger Federer & Rafael Nadal: The Lives and Careers of Two Tennis Legends – was in no doubt as to where to rank their rivalry.
"I think it can be said that it was the greatest rivalry in the history of tennis because they have played many times and the styles were opposite," observed Fest.
"A left-hander against a right-hander, a one-handed backhand against a two-handed one, an offensive player against a defensive player, although Nadal then added pepper to his game.
"The apparently cold Swiss, who was not so cold, against the apparently sanguine Spanish, who was not just that either. It was a rivalry with a great contrast.
"It is true that Nadal and Djokovic have played more times, and it is a much more balanced head-to-head, but what Nadal-Federer generated was pure happiness and if compared to the rivalries of the '70s, it was clearly superior.
"Nadal is Nadal because he is Nadal and Federer is Federer because he is Federer. But Federer is more Federer thanks to Nadal and Nadal is more Nadal thanks to Federer, of this there is no doubt because the two have pushed each other to new limits and have made each other better.
"When your rival is so good and beating them becomes so complicated, you have to look for alternatives and fine-tune the details."
But does Fest believe that Federer will go down as the greatest player of all time?
"It's very difficult to establish that," he added. "When you ask Federer or Nadal, they say that the logical thing would be to compare by era.
"It is very difficult to compare a player from 2022 with one from 1960 because there was a different physical preparation, other footwear, other materials, other racquets and other balls.
"I once asked Nadal how good he would be playing with wooden racquets and if that would harm him, or if Federer would gain so much advantage, and Nadal's theory is that the one who is good is good.
"So if he's good, one will pick it up with a wooden racquet to prove it.
"It can be said that Federer is one of the greatest of all time, and whoever wants to choose him as the best in history has all the arguments to do so."