Serena and Venus Williams will walk the slippery line between sisterly affection and athletic aggression for the 30th time on Friday, fighting for a place in the US Open last 16.

The superstar sisters' third-round clash is their earliest Grand Slam meeting in 20 years -- since Venus beat her younger sister in the second round of the 1998 Australian Open in their first career meeting.

"I never root against her, no matter what," Serena said.

"So I think that's the toughest part for me. When you always want someone to win, to have to beat them ... I know it's the same thing for her.

"When she beats me, she always roots for me as well. I think that's just the hardest part."

The possible all-Williams encounter had fans buzzing as soon as the draw was announced, but Venus could barely bring herself to discuss it after securing her passage with a 6-4, 7-5 victory over Camila Giorgi.

Venus's seven Grand Slam titles include a victory over Serena in the 2001 US Open final. But Serena has gone on to beat her in seven of eight major finals since as she built a 17-12 record over her elder sister.

That included a triumph in the 2017 Australian Open final, where Serena was already pregnant with daughter Olympia.

Venus won the most recent meeting, a third-round clash at Indian Wells in March, but Serena has since gained steam in her post-pregnancy comeback, reaching the final at Wimbledon.

Seeded 17th -- nine spots above her world ranking -- Serena is trying to avoid her first year since 2011 without a major title.

If she can pull it off -- and the signs improved with the first-round exit of world number one Simona Halep atop her quarter of the draw -- Serena will match Australian Margaret Court's all-time record of 24 Grand Slam singles titles.

It's not just heightened emotions that make facing Venus so hard, Serena said.

It's also the challenge of facing a player intimately acquainted with every aspect of her game.

"You have to almost become a different player," Serena said.

The very factors that make the match-up so daunting for the sisters are what make it so fascinating for fans.

"I'm sure they will hate it, both of them hate this," former world number one Victoria Azarenka said. "But I think it's going to be beautiful for tennis."