They first met in a grand slam -- the 1998 Australian Open -- and for world No. 1 Serena facing her old sister Venus never gets any easier.

"The only player in the draw I don't want to play, not only because she's my sister, but for me she's the best player," the 33-year-old Serena said of Venus, a former No. 1 who is now ranked No. 23 in the world.

"She has beaten me so many times," added Serena, who has defeated her sister 15 times in their previous 26 meetings, winning eight of their previous grand slam encounters.

"She's a player that knows how to win, knows how to beat me and knows my weaknesses better than anyone."

Beating Serena at a grand slam? Yes, it's possible

Venus looks to have her work cut out in Tuesday's match to get the better of her sister, even though at the U.S. Open their four previous meetings have been shared.

Serena's imposing game, force of will and hunger have made the American especially difficult to stop at tennis' four major tournaments, with the 33-year-old having a a winning percentage of close to 90% in the grand slams.

"When she is in those events is where she really turns it on," former world No. 4 Samantha Stosur told CNN.

Williams has, in fact, walked away from a major 39 times without hoisting the trophy but those defeats have been spread over 17 years and it hasn't happened since Wimbledon in 2014, when Alize Cornet shocked her in the third round.

If Serena reaches and wins Saturday's final, it will be her 22nd slam singles title, matching Steffi Graf's Open Era record and and will leave the American just two shy of Margaret Court's all-time mark.

The two sisters met at Wimbledon in July, where Serena claimed a comfortable win before going on to lift the title for the sixth time.

Two down, five to go for Serena

The 35-year-old Venus has won seven grand slams -- the 2000 and 2001 U.S. Opens and five Wimbledon titles, the most recent in 2008 -- but she recognizes she is very much the underdog.

"It's easier said than done," said Venus, who did beat Serena in the 2001 final in New York.

"Even though you're playing your sister you have to be prepared and focus. The preparation doesn't change."

Stosur, who holds the distinction of being both the last player to beat Serena Williams in a grand slam final and the last player to defeat her at the U.S. Open, both in 2011, said having the requisite amount of belief is key.

"if you have the opportunity, you have to play the point and try not to think about who is on the other end," said the Australian.