Match of the day:  Ons Jabeur (TUN) vs.  Zheng Qinwen (CHN)
It's already been a landmark US Open for Chinese tennis, with two players -- Zheng Qinwen and Wang Xinyu -- in the last 16 for the first time ever. Zheng, 20, has the opportunity to seal another milestone as she bids to reach her first Grand Slam quarterfinal against Ons Jabeur.
That wouldn't just be significant on a personal level. No Chinese player has made the last eight of a major since Wang Qiang at the 2019 US Open. Seven months later, Covid-19 hit -- and the Chinese cohort were among the hardest hit, having to deal with travel restrictions and closed borders for a longer period than most others.
Different players dealt with continuing their international careers in different ways. Wang Yafan, who upset Caroline Garcia in the first round, took seven months off the tour, enjoyed her new marriage and returned rejuvenated this year. Zheng, by contrast, was the first Chinese player to resume playing international tournaments, basing herself in Barcelona in 2021 and taking road trips across Europe as she climbed the rankings ladder.
After establishing herself as one of the most promising talents in the game in 2022, Zheng's 2023 has been a mixed bag. She cracked the Top 20 in May and won her first title in Palermo in July, but there have also been a series of minor injuries and missed chances that have set her back.
Two battling wins over Kaia Kanepi and Lucia Bronzetti have been testament to Zheng's determination to grasp this fortnight's opportunity so far. Jabeur presents a step up in challenge -- Zheng has yet to beat a Top 10 player at a Grand Slam -- but the Tunisian has appeared vulnerable so far.
Having struggled with illness at the start of the tournament, Jabeur has needed to summon all of her own grit to come through three tightly-contested rounds against Camila Osorio, Linda Noskova and Marie Bouzkova. The last of those, a 5-7, 7-6(5), 6-3 win, lasted 2 hours and 56 minutes.
But emerging victorious from these tribulations has enabled Jabeur to rediscover something important: her sense of fun on court. When the US Open began, the wounds from her Wimbledon final loss were still raw. Now, she's enjoying herself somewhat.
"This is a fun tournament," she said after beating Bouzkova. "Not fun starting being sick. I feel like this tournament is testing me and showing me that I have a lot of strength, that I can go really far and maybe have a great result here.
"I love fighting. I trust my body. I know I can go all the way playing three sets, no problem."
Zheng and Jabeur have only faced each other once, in the second round of Toronto 2022, when Jabeur retired trailing 6-1, 2-1 due to abdominal pain.
Also in action
 Daria Kasatkina vs.  Aryna Sabalenka
One player who hasn't needed to battle especially hard is Australian Open champion Sabalenka. That's particularly impressive considering that the World No.1 ranking has, once again, been on the line for her here. Sabalenka has conceded just 12 games in three matches so far, albeit without facing an opponent in the Top 50.
That dynamic changes in the last 16. Iga Swiatek's loss to Jelena Ostapenko on Sunday means that Sabalenka will officially take over the top spot after the US Open. The pressure of chasing it is off -- so will Sabalenka be able to swing even more freely? She'll face her highest-ranked opponent yet when she takes on Kasatkina for the seventh time. Sabalenka was a 6-3, 6-3 victor in the third round of Cincinnati last month, and leads the overall head-to-head 4-2 (though they are all square at 2-2 on hard courts).
 Madison Keys (USA) vs.  Jessica Pegula (USA)
Somewhat surprisingly, American compatriots Pegula and Keys -- born just a year apart -- have only played each other once. That was in last year's San Diego quarterfinals, which Pegula won 6-4, 7-5. For most of their careers, Keys has been in the upper echelon of the rankings and frequently in the conversation for major titles -- a position Pegula has only found herself in recently.
Both players had to come through three-setters against tricky opponents in the third round, Pegula overcoming Elina Svitolina 6-4, 4-6, 6-2 and Keys quelling Liudmila Samsonova 5-7, 6-2, 6-2. A win would put Pegula into her seventh major quarterfinal (all since 2021), and third of the year; Keys is bidding for her ninth major quarterfinal, and first at the US Open since 2018.
Peyton Stearns (USA) vs.  Marketa Vondrousova (CZE)
Two months ago, Vondrousova and Stearns played in the first round of Wimbledon on one of the smallest courts at the AELTC. Vondrousova came through 6-2, 7-5 and a fortnight later, was lifting the Venus Rosewater Dish as the brand new champion at SW19. She's brought that form into Flushing Meadows, dropping just 10 games so far to extend her winning streak at majors to 10 matches.
Stearns has also been on a steep upwards trajectory. The 21-year-old American told press after defeating Katie Boulter 6-4, 6-3 in the third round that she keeps having to reset her ranking goals after hitting them ahead of schedule. The latest of those is cracking the Top 50, which she is now guaranteed to do after reaching the second week of a Slam for the first time. Not bad for a player who, this time last year, was ranked No.332 and only just embarking on her pro career after a stint at the University of Texas.