Defeating one Williams sister is hard enough. Beating them both in the same tournament is a rare feat - but that is the prospect Kim Clijsters is facing.
The 26-year-old Belgian has set New York alight in her comeback to tennis as she bids to become just the third mother to win a Grand Slam title and the first since Evonne Goolagong in 1980.
Clijsters knocked out third seed Venus Williams in a fourth-round thriller on Sunday and she will next go up against China's Li Na.
A win there could set up a semi-final against defending champion and tournament favourite Serena Williams, should the American see off the challenge of Italy's Flavia Pennetta.
The win over Venus has been inspirational for Clijsters, proving to herself that she can still take on and defeat the very best.
"I know that Venus wasn't playing her best tennis, but there's a lot of other things personally that I'm taking out of this match," she said.
"That's just the mental strength that I kept focusing on and just trying to regroup after the second set.
"Those kinds of things. Reading the game a little bit, adjusting my game a little bit to a couple of weaknesses that I had out there."
Next up for Clijsters though will be Li, the 18th seed who is into the last eight at Flushing Meadows for the first time and who is playing some of the best tennis of her life.
"I've always had tough against her. She hits a great forehand," said Clijsters of the Chinese No.1 who has been a trail-blazer for her country in tennis over the last few years.
Li, for her part, said that she had realised a dream in reaching the last eight at the US Open. "I just hope I can win three more matches," she said unconvincingly.
Pennetta, a 27-year-old from Brindisi who this year became the first woman from her country to breach the WTA top 10, saved six match points in her semi-final win over Vera Zvonareva of Russia.
But asked if she could take her run one step further past Serena, Pennetta sounded dubious.
"I think she is the best player, you know. She's strong," she said.
"But I don't have anything to lose, so I can go on the court and enjoy and try to play my tennis and that's it.
"I just played one time against her in Miami two years ago. I lost 6-3 in the third and it was a pretty good match, so we will see."
Williams has looked imperious so far conceding just 17 games in four straight sets wins as she bids to make it three out of four Grand Slam titles this year.
Her only defeat came at the hands of eventual winner Svetlana Kuznetsova of Russia in the French Open quarter-finals and that was a desperately close three-setter.
She has already seen the Russian armada all but eclipsed with the defeats of Dinara Safina, Maria Sharapova, Elena Dementieva and Zvonareva, but she insists she is taking nothing for granted.
"I think everyone is playing so well," she said.
"Some of the seeds have been struggling all summer. You can't underestimate anyone. You have to be ready for every match."