PARIS -- They have won more Grand Slam singles titles (32 combined), more prize money ($120 million-plus) and spent more time ranked as the No. 1 player in the world (441 weeks) than any of their active contemporaries.
On Tuesday, they will each play a quarterfinal match here at Roland Garros, and though history says they have lost in some previous matches to their formidable opponents, there are no compelling reasons to think they won't advance to the semifinals.
Williams is in the midst of a 28-match winning streak, the third-best run on the women's side since the millennium. Federer won his 900th ATP World Tour match Sunday, a feat that hasn't been registered in more than 22 years.
Neither player seems terribly interested in the numbers they are amassing. There is a more important focus.
"I honestly don't even consider it a streak," Williams said after slaying Roberta Vinci in the fourth round. "For me, it's about winning the matches. At the end of the day, I just want to hold up the winner's trophy and whatever it takes to get there."
Federer sees things precisely the same way.
"I'm just happy I've been able to win a lot of matches throughout my career, really," he said in response to a question about bagging No. 900. "Give myself an opportunity over and over again. I love this game. These wins have definitely helped me continue working hard, be motivated. Being part of great matches, and that I can come out so often on the winning side is obviously amazing for me."
And, it must be said, for the rest of us, too.
Williams hasn't lost since Feb. 18, when she fell to Victoria Azarenka in a three-set final in Doha. Twenty of those 28 consecutive wins have come on clay. She meets Svetlana Kuznetsova, a quirky performer who has managed to win two major singles titles.
The 27-year-old Russian admits she has problems getting motivated for the smaller events, but when she arrives at the Grand Slams, suddenly she smells the espresso. Kuznetsova won the title here in 2009, beating Serena in a spirited quarterfinal that went the distance. More recently, however, Williams flogged Kuznetsova last month in Rome, 7-5, 6-0.
"She's the best in the world so far," Kuznetsova said, after dispatching Angelique Kerber in the fourth round. "She's been playing unbelievable tennis. She's just a fighter. But I believe I have the game and my good days, as well. Let's cross fingers I will have a good day that one."
Serena, who has lost all of 10 games in the eight sets she has played, was just as respectful in her comments about Kuznetsova.
"It will be a good match," Williams said. "I mean, the last time we played here she won, so that will probably get her pumped up and she's on a comeback. She has nothing to lose and everything to gain.
"It will be a good job for me to see how I go there."
It has been a decade since Williams reached the semifinals here -- she lost to Justine Henin -- and 11 years since she beat her sister Venus in the final for her only French Open title.
The Swiss champion had won each of his 10 sets here until Frenchman Gilles Simon surprised him by taking a two-sets-to-one lead in their fourth-round match Sunday. Federer ran off seven straight games at the end of the fourth set and the beginning of the fifth and eventually closed out Simon.
Federer, who may have tweaked his bad back during a nasty fall, had 60 winners and 56 unforced errors in the match and will have to play better against Jo-Wilfried Tsonga, whose game he knows quite well.
Federer has played the Frenchman no fewer than 12 times and won nine. And while Tsonga took their 2011 Wimbledon quarterfinal in a five-set thriller, Federer won the other three Grand Slam matchups, including this year's Australian Open quarterfinal match that went, predictably, five sets.
"He's already beaten me in the past twice, I believe," said Federer, missing the mark by one. "I think that's enough for him to believe he can beat me. He doesn't need any additional motivation. I know he can beat me and he knows he can beat me.
"But I'm going to make sure that doesn't happen."
Goodness, was that a guarantee?
Federer and Tsonga have developed a genuine fondness for each other. They met twice during Federer's six-match tour of South America last December. They met in Sao Paulo, Brazil and, one week later, in Bogota, Colombia, with Federer winning four of five exhibition sets. But watch the YouTube clips of their clowning camaraderie, their soccer skills with a tennis ball, the smiles that they bring out in each other and you will understand their relationship isn't typical.
"Obviously, it's a big challenge playing him here in Paris," Federer said. "He's a great friend of mine. We had a great tour together in South America.
"I think we're both looking forward to this match."