If she beats Sloane Stephens on Thursday to reach Saturday's final, Australian Open and Wimbledon runner-up Williams will reach her third Slam final of the year, a feat she so far has only achieved in 2002.
Stephens, meanwhile, defeated Anastasija Sevastova, of Latvia, 6-3, 3-6, 7-6 (4), to reach a major semifinal for the first time since 2013.
Her only previous Grand Slam semifinal was at the Australian Open four years ago. It was a thriller before the thriller, with Sloane having to come from a break down in the third set to pull off the upset (on paper), 6-3, 3-6, 7-6 (4) in a tiebreaker.
Once looked as the successor to the Williams sisters, Stephens' career has idled, and she was completely on idle for almost 11 months with a foot injury that required surgery.
Neither showed particular boldness or ferocity as the match proceeded to a tiebreak, though the Latvian certainly displayed a particular knack with her confounding drop shot.
Most of Venus Williams' American rivals from the early 2000's have long since retired from tennis. She returned to the tour at the French Open in May, losing in the second round there and at Wimbledon. None of those three men remaining on the bottom half of the draw has ever played in a major final.
Federer set aside an uncharacteristic 68 unforced errors and what appeared to be a bad back, managing to pull out his second five-set victory in a row at the U.S. Open by coming back to edge Youzhny 1-6, 7-6 (3), 6-4, 4-6, 6-2. "When I wasn't playing, like, of course I loved my time off, but when I got back to playing tennis, it was, like, this is where I want to be".
The 2017 U.S. Open marks 20 years since Williams debuted at 17 in world-famous hair beads and toiled her way all the way to the final against Martina Hingis. "Anybody's game. I just have to focus on myself and, yeah, that's all I can really do".
"I also think it would be wonderful if Venus wins a Slam the same week her sister Serena has a baby", Evert said on ESPN.
Kvitova also drew first blood in the decider, using her crafty backhand to devastating effect to break for a 2-1 lead, but a double fault on breakpoint in the sixth game allowed Venus to level.
The elder of the Williams sisters will start as the favourite to progress to her first final in NY since then.
"It's pretty awesome", said Keys after completing the foursome with a 6-3, 6-3 victory Wednesday over Estonian qualifier Kaia Kanepi.
"Man, I am hoping we can make it four, how awesome would that be", said Stephens.
"It's like a dream come true", Muguruza said in a video she posted on Twitter.
"I had my chances", Kvitova said.
The other men's quartefinal on Tuesday will pit Pablo Carreno Busta, the 12th seed, up against Diego Schwartzman, the 29th seed.
The final stages of this year's US Open have produced some passionate tennis and is replete with inspiring stories.