And much like Billie Jean King, there's no reason to believe the woman wouldn't win in this specific match as well. "Clearly she's seen me play many, many times".
It's the first all-American US Open women's semi-finals since 1981 and the first at any Slam since Wimbledon in 1985. "Maybe it's like some Jedi mind trick", said Vandeweghe, who still did not hide her emotions on court, such as when she cracked a racket against the ground after a second consecutive double-fault in the opening set.
With her loss to American Coco Vandeweghe in the US Open quarterfinals Wednesday, Karolina Pliskova also will lose the No. 1 ranking. Williams had 17 unforced errors in the set and won only 13 points, just seven on her serve. You'd like to see Vandeweghe clean up on 32 unforced errors she had in her last match, but this is easily the best we've seen her play yet.
Keys has found her form on the U.S. hard courts, claiming the title in Stanford and reaching the third round in Cincinnati where on both occasions she beat Vandeweghe, her opponent in Thursday's semi-finals.
"You guys are the best", Vandeweghe said. "It's obviously going to be tough", Stephens said.
Victory over Keys would make her just the third qualifier to reach a semi-final at the majors after Christine Dorey at the 1978 Australian Open and Alexandra Stevenson at Wimbledon in 1999.
"It's really unbelievable, not only for myself but just for American tennis", said Keys. "But now I'm afraid to, like, play in the sun because I haven't done it in two weeks, so".
Williams, the most successful female player of the Open Era, gave birth to her first child last month but while the 35-year-old is on maternity leave her compatriots Keys, Coco Vandeweghe, Sloane Stephens and older sister Venus Williams have shone on home soil.
Should Nadal and Federer both win, then they will meet for the first time in NY in Friday's semi-finals.
"I'm really excited and proud of all of us for getting this far", Keys said. Where Clijsters came into the tournament unseeded, Stephens barely owned a world ranking, having stood at No 957 as recently as the end of July. "Clearly, I mean, I'm on that schedule already, might as well stay on it", she joked. She lost her first match back, and then her second, as well, at a tournament in Washington.
"It's such a wonderful feeling, because you put in so much work to be able to succeed at these moments", said Williams, on the verge of making three Slam finals in a year for the first time since 2002. "And then, yeah, look, there, semifinal, semifinal, semifinal".
"I knew I had to rise to the occasion and I'm just really happy to be in the final".
Sevastova, who beat Maria Sharapova in the fourth round, used all manner of mixed speeds and angles against Stephens, often winning points with the help of drop shots.
This is her sixth trip to a Grand Slam quarterfinal, but first since Wimbledon in 2013.
Stephens is 4-0 in WTA finals, having won titles in 2015 at Washington and past year in Auckland, Acapulco and Charleston.
"I hung tough, gritted it out", said Stephens.
Del Potro showed no ill effects from his 3 1/2-hour, five-set comeback victory in the fourth round or the illness that's been bothering him and had the 6-foot-6 (1.98-meter) Argentine coughing into a towel late in the second set.
"He's been playing better and better every day", said Carlos Moya, the 1998 French Open champion who is helping coach Nadal.
Each player received a trophy and a $5,000 donation to the charity of his or her choice.