Yes, the US Open lost a lot of star power when Naomi Osaka and Ashleigh Barty departed earlier in the event. 

But that vacuum did not last long. Up stepped Emma Raducanu, at 18-year-old ranked No.150, and Leylah Fernandez, 19, who is No.73 in the world. And while their talent was never in question, the idea of them reaching the final four at one of the biggest events on the calendar has to feel like a dream. 

"I've imagined myself playing on every tournament, every Grand Slam, at the biggest stage," Fernandez said after her last match. "When I was younger, since I used Justine Henin as a great example, I would imagine myself playing against her. I would also imagine myself playing against Serena and Venus, and the past few years playing against Osaka in a big tournament.

"When I was younger, I've always seen myself being in a big stadium in front of so many people and just having fun on the court."

Can the teenagers continue to play at the level they have been for nearly two weeks in New York? What adjustments do they need to make against their more established opponents in No.17 Maria Sakkari and No.2 Aryna Sabalenka?

And most pressing, who will advance to the US Open final Saturday for a shot at the title? Our team breaks it down. 

No.17 Maria Sakkari vs. Emma Raducanu

Key for Sakkari: Be bold, be brave

Maria Sakkari is known for her warrior qualities, but her fighting spirit is as practical as it is innate, as it leads to a more energized and effective game. Sakkari knows that bravery is the key to pulling her through the biggest matches. 

“[Bravery] is something that I've been working with [coach Tom Hill] since the end of last year,” Sakkari said Monday. But after a middling hardcourt swing leading into the US Open, Sakkari said she “lost my identity, that's how I called it. I lost myself, part of myself.

“I found a way to come back and feel again what I felt out there today. It's just a process in practice and everything that has helped me to be more brave.”

At 26, Sakkari trusts herself and knows what she has to do to excel. That was evident in her 3-hour, 30-minute late-night victory over Bianca Andreescu, which ended after 2:00 a.m. Tuesday morning in a match in which she hit more winners than the former US Open champion (46 to 39).

“I was more brave after a certain point of the match, then got my energy level a little bit higher,” Sakkari said following that win. “I said, 'Maria, if you want to win this match, you have to go for it’. ... I just wanted to be more brave, and that's what I did.”

Not many players are as speedy and strong on defense as Sakkari, but when she is bolder on big occasions, that’s when her game levels up. The pattern continued with her win over Karolina Pliskova in the quarterfinals. At one point, Sakkari captured 22 straight points on her serve and all together, she won a stunning 92 percent of points behind her first serve.

With that mentality, Sakkari moved into her second Grand Slam semifinal of the season. She will need to maintain that mindset against her free-swinging semifinal opponent, Emma Raducanu. But Sakkari has learned exactly what kind of fighter she needs to be, and if she executes courageous play, she has an excellent shot at a first Grand Slam final. -- Jason Juzwiak

Key for Raducanu: Keep her head down

A series of successful results can create momentum – and the belief, the burgeoning confidence that goes with it. Wednesday, Raducanu simply had more against reigning gold medalist Belinda Bencic. Down 1-3, Raducanu rallied to win 11 of 15 games and landed in the semifinals opposite No.17 Maria Sakkari.

The phenom from Great Britain has kept the statisticians scrambling. The best one: She’s the first qualifier of either gender to reach the US Open semifinals in the Open Era, and only the fourth to reach any Grand Slam semifinal.

“It’s the first time I heard today that I was the first qualifier to make the semis,” Raducanu told reporters. “I had no idea before that. Yeah, I’m not here to chase any records right now. I’m just taking care of what I can do on the moment and on the match ahead.”

Keep in mind, this is only her fourth tour-level main draw – and Bencic was the first Top 40 WTA player she’d ever faced. And somehow Raducanu has won all 16 sets she’s played, with only one (7-5 in qualifying, against Mariam Bolkvadze) being even slightly concerning.

So, for Raducanu this astonishing, unlooked for semifinal demands more of the same. Keep your head down, focus on every point and continue playing with that confidence you’ve built up around you like one of England’s great medieval fortresses.

“I didn’t expect to be here at all," Raducanu said. I mean, I think my flights were booked at the end of qualifying, so it’s a nice problem to have. Out there on the court today, I was saying to myself, 'This could be the last time you play on Ashe, so might as well just go for it and enjoy everything.’”

Keep that momentum going. -- Greg Garber

No.2 Aryna Sabalenka vs. Leylah Fernandez

Key for Sabalenka: Maintain glass-half-full approach

Following a straight-sets victory over Barbora Krejcikova to reach the US Open semifinals, Sabalenka, unhappy with her play, headed directly to the practice court.

“For real, I feel like I didn’t move well today and I needed extra balls kind of a little bit to move, a little bit to feel my legs, to feel the court,” Sabalenka said. “Also my serve was really I wouldn’t say terrible but was really bad today. I was trying to find the rhythm. So, yeah, I needed these extra balls today.”

This is the critical attitude she needs to carry into her Thursday semifinal against 19-year Leylah Fernandez.

The 23-year-old from Belarus has displayed a growing maturity this season and it’s carried her to new heights. She had played in 14 majors and never reached a quarterfinal – until Wimbledon. Now, she’s done it twice in a row.
    
In her five matches, she’s stroked a characteristically healthy number of aces (29), but there have also been 35 double faults. And yet, it hasn’t cost her. After dropping a second-set tiebreaker to Nina Stojanovic in the first round, she came back with a 6-0 third – and has won nine straight sets.

In recent years, Sabalenka sometimes lost focus when one of more of the components of her game went south.

“When something is not working well, what I actually improved in, I’m trying to find something else to win the game,” Sabalenka said. “I’m really happy that I’m not really focusing on the problem. I’m focusing on what should I do to win this match.

“I’m not really panicking or stressing about it. I’m just trying to stay smart and professional.”

She’s ranked No.2 in the world and one of four semifinalists looking to break through and win her first Grand Slam singles title. Why not Sabalenka?

“Where I am?” she said playfully. “I’m in the semifinal of US Open, this I know. Yeah, second semifinal, and hopefully I can keep it up.” -- Greg Garber

Key for Fernandez: Keep up that serving percentage

No player has had a tournament rivaling Leylah Fernandez's. The 19-year-old Canadian has cut through an absurdly tough draw like a hot knife through butter. Her perfunctory wins over Ana Konjuh and Kaia Kanepi in the first two rounds laid the foundation for a string of marquee victories that saw the No.73 beat a trio of veteran players at their own games. 

"I think we're all just super hungry to make a difference in the tennis world," Fernandez said, referring to the surge of teenagers at this year's US Open. "I've known a few of them from the junior tours. We've always talked about and joked around that we're going to be in the WTA Tour, we're going to be on the big stage together. I'm just glad that we're doing such great jobs and doing just that.

"We want to make a difference. We want to make an impact in tennis. This tournament just proves how well we're adapting to everything."

Adaptation has been Fernandez's calling card in New York. Against defending champion Naomi Osaka, she flummoxed the World No.3 with her lefty-serve. Against former No.1 Angelique Kerber, Fernandez out-counterpunched the 2016 champion. To earn her second Top 5 win of the tournament, she beat an in-form Elina Svitolina by playing gritty defense to hold off the Chicago champion's late comeback.

Three high-quality wins won with three distinct styles. Fernandez's tactical versatility is what has stood out in her incredible run to a first major semifinal. But her task of unwinding Aryna Sabalenka is a tough one. The power differential is plain to see, but Fernandez should take confidence from her effort against Osaka, where she converted 80% first serves in and faced just five break points. Fernandez will need to replicate that effort to keep Sabalenka's devastating return away from her second serve.

Fernandez has been masterful in how she has used the crowd to her advantage throughout the tournament. New York has been vociferously backing the underdogs all tournament, and the fans will undoubtedly get behind Fernandez's scrappy game as she fights to absorb and redirect Sabalenka's explosive groundstrokes. Sabalenka was in a similar position in the second round at Wimbledon, where Britain's Katie Boulter nearly rode the wave of Centre Court support to an early upset. If Fernandez can get the crowd going again and keep the scoreboard pressure on the Belarusian, Sabalenka could lose patience, overpress and leak errors. -- Courtney Nguyen