When Roger Federer was a month past his 20th birthday he was ranked No 12 and had one senior title to his name, having beaten Julien Boutter, the world No 67, in the final of an indoor event in Milan.
Rafael Nadal was 20 last month. He has been world No 2 for a year and has won 17 titles, including two French Opens. He holds the world record for successive wins on clay, and has beaten Federer, the world No 1, in six of their eight meetings, including four finals this year.
Federer's four-set victory over Nadal in the Wimbledon final on Sunday proved that he remains the world's best player on grass, but the question of whether he can go on to rewrite the record books is probably in the hands of the young Spaniard as much as it is down to himself.
Four successive Wimbledon crowns have already elevated Federer, at just 24, to joint fifth on the All England Club honours board, alongside Rod Laver, Anthony Wilding and Reggie Doherty. Ahead of him are Bjorn Borg and Laurie Doherty, with five titles to their name, and Pete Sampras and William Renshaw, with seven.
Only five players have won more Grand Slam titles than Federer, who has eight (Sampras tops the list with 14). Federer has been at the top of the world rankings for 128 weeks in succession, the third-longest reign since they were launched in 1973, behind Jimmy Connors (160 weeks) and Ivan Lendl (157 weeks).
How long before Nadal starts to join some of those lists? "I'm only 20 and Roger's 24, so maybe I can improve in the future," Nadal said as he reflected on Sunday's final. "When I was playing my best tennis the match was very close.
"I wanted to play well in this final because it was important for my belief in my ability to win here in the future. I know that I can beat Roger and it was important for me to believe that I can beat him here as well."
Federer enjoys the rivalry. "When we play so often in finals I think it adds something to the game," he said. "He's up-and-coming. I used to be the youngster but now I'm getting older. It's a great rivalry."
Nadal's progress on grass is all the more commendable considering his lack of experience. As a junior he played in only one grass-court tournament, losing in the Wimbledon semi-finals. He arrived at the All England Club last month having won only three matches in his two previous senior appearances there and with just two other wins on grass under his belt, against Mardy Fish and Fernando Verdasco at Queen's Club four weeks ago.
The Spaniard has learned much over the last month. "I know more about how to move on a grass court, about strategy, how to serve, how to play more aggressively with my forehand," he said. "I need to change a bit, to hit the ball a bit more slowly, without a lot of top-spin."
Federer has already noticed the latter change. "I was surprised how hard he started to hit the ball in the end," he said. "He started to stop hitting the balls with spin and instead was just swinging more freely, especially his backhand. That makes it more difficult because all of a sudden he's much more dangerous."
The world No 1's coach, Tony Roche, believes his man can only get better. "I know he wants to improve," he said. "As long as he has that attitude, there's no reason he can't."
If Nadal is the undisputed king of clay and Federer reigns on grass, what of hard courts and the forthcoming American season? Nadal has won two of their three meetings on the surface, including the most recent in Dubai earlier this year, but he has not gone beyond the third round in three visits to New York, where Federer will be going for his third successive US Open title.
Moreover, other players, such as Ivan Ljubicic, James Blake and David Nalbandian, will see the US Open as their chance to make some ground on the top two. Some of Nadal's contemporaries are also starting to break through. Marcos Baghdatis' run to the Wimbledon semi-finals took him into the top 10 for the first time in yesterday's new ranking list and proved that his appearance in the Australian Open final was no fluke.
As some of the older guard slip down the order - Andy Roddick, at No 11, is out of the top 10 for the first time since 2002 - three 19-year-olds are climbing swiftly. Gaël Monfils is at No 23, Novak Djokovic has climbed 28 places to No 35 in the last six weeks, while Andy Murray's rise to a career-high No 36 has seen him reclaim British No 1 status from Greg Rusedski.
Murray, who is the top seed this week in the grass-court tournament at Newport, Rhode Island, has always said that he expects to perform best on American hard courts and won the US Open junior title two years ago.
For the moment Federer is the man they all look up to, but, as Nadal has shown, men's tennis can be a rapidly changing world.