Andy Roddick was just a few weeks beyond his 21st birthday when he won the U.S. Open and became the world's No. 1-ranked player. Roger Federer, Rafael Nadal and Novak Djokovic were all 24 when they won three majors in a single season for the first time.
How to explain the strange and wondrous ascent of Mardy Fish, who will turn 30 in less than a month? At the advanced age of 29, in his 12th season as a professional, Fish finds himself in the elite eight for the first time. He is the eighth and final seed in the Barclays ATP World Tour Finals that begin in London on Sunday. Fish, described as a debutant in the ATP press release, plays Nadal in his first round-robin match Sunday night.
The others in his group? Guys named Federer and Tsonga.
"There are not too many holes there -- apart from me," the self-deprecating Fish said on a conference call earlier this week. "I still don't feel like I really belong, but I'll try to enjoy it.
"I know I'm guaranteed at least three matches. And I'm guaranteed to play two of the best players of all time, which is pretty daunting."
Fish will not be anything close to the leading storyline at The O2. No. 1 Djokovic, he of the delicate right shoulder, will try to close out his best season with a flourish. No one is quite sure what we'll get from No. 2 Nadal, who apparently is still nursing a foot injury and focused on the Davis Cup final that will conclude the men's season. The two hottest players coming in are No. 3 Andy Murray, who has won 18 of 19 matches, and No. 4 Federer, who has won two titles and 12 straight matches. Federer is looking for his record sixth year-end title, which would break a three-way tie with Pete Sampras and Ivan Lendl.
All things being equal, Fish would have a chance to win a match or perhaps even two. The indoor court is fast, much to his liking.
"I'm sort of taking the approach that I'm excited to be part of this thing," he said. "You know, Roger has won this tournament five times. I can't really imagine that kind of success. There's zero pressure [on me]. It's not out of the realm that I can get to the semis."
Problem is, all things aren't equal. Fish has retired from two of his past three matches with a left hamstring injury. Told Federer had said that Fish came into the event a tad "weary," Fish said, laughing, "What does Roger know?
"It's no secret. I have small problems with my fitness. I'm going around the clock to recover this thing. Hopefully, I'll be close to 100 percent. I'm ready. I don't want to give up my spot to anyone."
The key word was hopefully; Fish, candidly, did not sound overly optimistic. He has been employing a regimen of ice, ultrasound, electric stimulation, massage and rest since retiring from his second-round match against Juan Monaco in Paris. The season, he said, noting the injuries to Djokovic and Nadal, is relentless.
"You play 10 out of 12 months, so we only get five or six weeks off," Fish said. "Your body -- eventually things go, whether it's mental or physical. It's such a big event. Fatigue won't be an issue. You can see the finish line."
Federer, the defending Barclays champion, meets Jo-Wilfried Tsonga in Sunday's day session, a delicious matchup. It's their seventh meeting this year; Federer holds a 4-2 advantage. After losing to the Frenchman, memorably, at Wimbledon and then in Montreal, Federer has won the last five sets between them, at the U.S. Open and this past week in the Paris final.
"I have had some really tough losses this year," Federer said in Paris, "but I kept believing that still the year wasn't over, I can still finish this year on a high, which that proves to be the case. Now I still have a massive highlight coming up in a week's time."
The uncertainty concerning Djokovic (he plays Tomas Berdych on Monday) and Nadal's physical condition leaves the tournament wide open.
"I don't feel like 5, 6, 7, and 8 have no chance," Federer said. "I definitely think they have a chance to go deep, like  with del Potro and [Nikolay] Davydenko making the finals. I definitely could see something happening as well like that this year.
"Let's assume that everybody is in great shape. I think it's going to be an interesting couple of groups, because everybody can beat everybody I think again."
One thing to keep in mind: The big four has won exactly one-third of the ATP's events this year. In fact, when one or more of them is entered in a tournament this year, one of them came away with the title 21 of 22 times. Robin Soderling was the only one to crack the code, in Rotterdam.
As Fish pointed out, his story has been well-chronicled. After suffering a knee injury at the end of last season, he completely changed his lifestyle, vowing to try his hardest to become fit in the few years he had left on tour.
He has a record of 43-22, won his sixth title in Atlanta and reached two other finals. More importantly, he reached the quarterfinals at Wimbledon -- the third major quarter of his career -- and rose as high as No. 7. He's spent 28 weeks in the top 10 and, by the sound of things, doesn't intend to go anywhere.
"With age comes experience," Fish said. "That's where I sit. I don't feel like when I turn 30, I'll be going backwards."
Greg Garber is a senior writer for ESPN.com.