"What's most impressive for me is how she won so many Majors after she turned 30"- Martina Navratilova on Serena Williams' longevity

Martina Navratilova appeared in the seventh episode of "The GOAT: Serena", a podcast about Serena Williams run by former players Chanda Rubin and Zina Garrison. The podcast hosts distinguished names from within and beyond the tennis community who discuss various aspects of Williams' career.

A tennis great herself, Navratilova was asked what impressed her the most about Serena Williams' career. Navratilova pointed to the fact that Williams has continued to win Grand Slams since turning 30.

"What's most impressive to me is how she won so many majors after she was 30 because it gets harder as you grow older," Navratilova said.

The 18-time Major winner went on to talk about the most significant rivalries from Serena Williams' career thus far. She claimed that former World No. 1 Justine Henin and Venus Williams were Serena Williams' biggest rivals. Serena leads the head-to-head 8-6 against Henin and 19-12 against her sister Venus.

"I think the closest rival would be Justine Henin and of course her sister, Venus [Williams]," Navratilova said. "There were a lot of contenders; Victoria Azarenka looked like she was heading that way but that fizzled out. She got injured and then got pregnant. So yeah Venus and Justine, that's it.

Venus Williams and Serena Williams at the 2017 Australian Open

The 64-year-old further highlighted that having a strong rival can benefit the players. During her time as a professional, Navratilova found a fierce rival in Chris Evert. The duo, who shared 32 Grand Slam titles between themselves, battled a whopping 80 times on the court.

In that context, Navratilova emphasized that rivalries help players improve their game.

"Yeah I mean it pushes you," she continued. "You can only be as good as your opponent. You can't hit great shots if you're not pushed into hitting great shots. With rivals you have to strive to improve, you can't just hold onto what you have you have to do better than that."

"So Chris and I did that with each other. Maybe we would've won more without the other but probably wouldn't have been as good. So that kind of paid off," the American added.

For Djokovic, Rafa, Roger and Serena Williams, it's all about the Majors: Martina Navratilova

Chris Evert and Martina Navratilova

During the course of the conversation, Martina Navratilova, who won a whopping 59 Grand Slam titles across disciplines, also explained how tennis has evolved over the years, Navratilova said regular tour events during her time were "huge" and carried a similar amount of prize money as the Slams, which is not the case today.

Modern-day players have the luxury of focusing on just the Majors, which are the most rewarding tournaments. The Prague-born added that the Majors are the biggest priority for players like Novak Djokovic, Rafael Nadal, Roger Federer and Serena Williams, who often treat lower-tier events as "paid practice".

"It's not a coincidence that right now we have the four people who have won the most [Grand Slam] titles other than Steffi Graf. Novak Djokovic, Rafa, Roger and Serena Williams. That's all tennis has become; it is all about the Majors. The other tournaments are basically paid practice," Martina Navratilova said.

"I would play 70-80 matches every season in singles and I would play doubles as well. The mentality was different and of course the tour was huge. The tour had a bigger prize money than the majors. Evonne Goolagong won 12 thousand dollars for winning the Australian Open, and then Chris Evert won the San Fransico tournament two weeks later and got 10 thousand dollars," she added.