Andy Roddick feels matches between Venus and Serena Williams were "awkward", Rosie Casals says Serena had "hidden greatness"

The "GOAT: Serena" podcast, run by former players Chanda Rubin and Zina Garrison, delves into Serena Williams' most prominent rivalries and career highlights.

The latest episode featured a number of notable guests from the tennis community, including former World No. 1 Andy Roddick, erstwhile pro Rosie Casals and Hall of Fame coach Benny Sims.

Roddick, winner of the 2003 US Open, spoke about how the early matches between Serena Williams and Venus Williams seemed "awkward," since neither player could truly enjoy the result. The first of the Williams sisters' 31 encounters took place at the 1998 Australian Open, which Venus won in straight sets.

"Those matches were kind of awkward for a long time," said Andy Roddick. "Maybe they know each other's weaknesses so they can attack them a little bit more, I'm sure there's some sort of chess-match explanation for it, but the matches kind of always felt awkward."

Roddick said such matches often ended with a bittersweet feeling with close to no celebration from the victorious sibling.

"Even at the end there was no celebration, it was as if they were happy for half a second and then they both had that face where they'd eaten a bad piece of food. So it was happy but sad at the same time. I am better off talking about their feats apart from how great their relationship is, but the matches were probably less than we'd expect from two players of their caliber," he added.

Serena Williams registered her first win over sister Venus at the Grand Slam Cup in Munich in 1999

Rosie Casals, one of the earliest activists for equal prize money, also gave her thoughts on the 23-time Major champion. The 73-year-old described Serena Williams as a warrior who can never be counted out.

She further discussed how there was "hidden greatness" within Serena ever since she first stepped on the court.

"I see her as a warrior," Rosie Casals said. "She's a great competitor because you can never count her out. You never know who's stepping on the court; is it somebody who's very good or better than that maybe great."

"There was some hidden greatness in Serena; she was quiet and very focused. Her sister went first and she followed and then she took over. Venus was too happy to let her do so I don't think it could ever be the other way around," she added.

"From a movement standpoint, Venus and certainly Steffi Graf were better than Serena Williams"- Benny Sims

Serena Williams at Wimbledon 2010

Benny Sims, former coach of Chanda Rubin, also gave his insights into Serena Williams' early rivals. He heaped praise on Venus Williams' powerful first serve, which he believes was as impressive as Serena's during the first half of her career.

He further expressed his appreciation for Justine Henin's versatility and Steffi Graf's on-court movement.

"Venus, from very early until the middle of her career, her first serve was certainly as formidable as Serena's," Benny Sims said. "Justine Henin was probably more versatile - she had a beautiful topspin, but it was her slice that got her out of trouble and made her so dominant on clay."

"Probably from a movement standpoint, Venus and certainly Steffi Graf where I think better than Serena Williams. I'll also have to give credit to Monica Seles and Martina Hingis' ball control," he added.