"I was sprinting and Serena Williams was participating in a marathon" - Andy Roddick on the secret behind Serena's longevity

Serena Williams' list of achievements on the tennis court has made her one of the most recognizable athletes in the world. The American won her first Major title at the US Open back in 1999, and more than two decades later she is still in the conversation when it comes to Slam contenders.

Former men's World No. 1 Andy Roddick recently spoke about the longevity of Serena Williams' career and her legacy in American tennis. Roddick has known Williams from a young age, having trained with her at Rick Macci's tennis academy in Florida.

In conversation with former tennis players Zina Garrison and Chanda Rubin - hosts of a weekly podcast - Roddick marveled at the level of self-confidence that Williams has shown over the years. According to Roddick, that self-confidence is what has has helped the 39-year-old pace her career a lot better than her contemporaries.

The American further credited Williams' ability to "show up" and play her best tennis after long breaks - without needing a lot of match practice - for her long-standing dominance.

"Serena would just show up having played just five times since the month of October, she would show up in Australia and play herself into the tournament because you're that great," Roddick said. "I'm struggling to survive even though I'm No. 3 in the world, it's just different gravy."

"But it takes a different level of self-confidence, and we've also seen this from Roger [Federer], where it's like I'm gonna take six months and come back and be refreshed," he added.

Serena Williams and Andy Roddick have known each other since their time togehter at Rick Macci's tennis academy

Andy Roddick also highlighted the difference in the manner that he and Serena Williams approached their careers, making use of a "sprint versus marathon" analogy. The American was focused on getting immediate results because he believed he had a limited window to play at his physical peak, but he now acknowledges that that approach was "short-sighted".

Roddick also claimed Serena Williams trusted her abilities and had a "process going the entire way", something that he didn't fully understand at the time.

"07' and 08', I'm training as hard as I possibly can everyday, trying to get every advantage," Roddick said. "I was sprinting and [Serena was] participating in a marathon of a career."

"I was short-sighted, I was insecure about my ability," he continued. "[Serena] would play two tournaments in the year and I would say, 'What are you doing? You're leaving it on the table. You only get this much time as a pro athlete and you've got to focus.' It was the common take back then. But if we have to admit it, we'd have to say that we were wrong. She had a process going the entire way."

Venus and Serena Williams were everything for US tennis: Andy Roddick

Serena (L) and Venus Williams at the 2018 US Open.

On being asked about Venus and Serena Williams' impact on American tennis, Andy Roddick "regurgitated" what several other players, women in particular, have said about the two.

Roddick claimed that getting a chance to share the court with his idols was a big highlight of his career, and that it must have been the same for the young female players who grew up idolizing the Williams sisters.

"I mean, you don't have to take my opinion on this," Roddick said on being asked about the impact of the Williams sisters on tennis. "I would just regurgitate what Sloane [Stephens] says, what Madison [Keys] says, all of the women players that we're now talking about, out of their mouths 'They were everything, Serena and Venus'."

"I had that moment of crossover, where I got to play my idols," the former World No. 1 continued. "It was phenomenal, beating Pete [Sampras] one time and all of a sudden, I'm going, 'Wait a minute, I beat one of the best players of all-time'. And having those moments, even practice with Serena, whatever it may be, makes you feel like you can do something."