NEW YORK, NY, USA - The field in Flushing Meadows has now been whittled down to the last 16 and among those looking to claim a place in the quarterfinals are defending champion Serena Williams, Agnieszka Radwanska and former finalist Jelena Jankovic.
Sunday, Fourth Round
Arthur Ashe Stadium
 Serena Williams (USA #1) vs.  Sloane Stephens (USA #16)
Head-to-head: series tied 1-1
Revenge is a dish best served cold, or so the saying goes. In January, Sloane Stephens scored the biggest win of her career when she knocked Serena Williams out of the Australian Open. Seven months later, the World No.1 gets her chance for payback.
From the moment the draw was made, the Fed Cup teammates' potential last 16 meeting immediately caught the eye. Stephens, however, very nearly failed to make it there after coming within a game of defeat in the first round against Mandy Minella. Since then, it has been plain sailing, with Stephens dropping just six more games during comprehensive wins over Urszula Radwanska and then Jamie Hampton.
Williams has been equally impressive in the early salvos of her title defense, wasting little time or energy in routine wins in her opening three assignments. Stephens, though, who is one of the few players on tour with comparable speed and athleticism to Williams, is likely to present a trickier proposition and has saved her best tennis this year for the majors, reaching the fourth round or better on each outing.
 Li Na (CHN #6) vs.  Jelena Jankovic (SRB #12)
Head-to-head: series tied 4-4
While Li Na may have arrived in the last 16 with little fanfare, it is not for lack of trying. Despite turning 31 earlier this year, Li, who has been rejuvenated since teaming up with Carlos Rodriguez, has been pulling out all the stops in the first week and was moving, serving and striking the ball better than ever against Laura Robson in the last round.
Jelena Jankovic's performances have not been quite so spectacular, but she will be similarly pleased with her progress, having not dropped a set thus far. Remarkably, this is Jankovic's deepest run in New York since finishing runner-up in 2008, and, in light of recent form, she will be confident of hanging around a while longer.
In Cincinnati, Jankovic laid the blueprints for overcoming Li with back-to-back wins over three fearsome baseliners, Sabine Lisicki, Ekaterina Makarova and Sloane Stephens. The players have split their eight career meetings, although the past three have gone to Jankovic, including this year on the clay in Rome.
 Agnieszka Radwanska (POL #4) vs.  Ekaterina Makarova (RUS #25)
Head-to-head: Radwanska leads 3-0
For a player that has won nine hardcourt titles, Agnieszka Radwanska's record at the US Open is somewhat disappointing, with a handful of fourth round appearances the highlights from her seven previous visits.
Last year, a weary Radwanska finally ran out of gas in the last 16 against Roberta Vinci, yet despite another busy summer, she looks far fresher this time around. Her preparations for Flushing Meadows went well, with deep runs in Stanford, Carlsbad and Cincinnati, and she finished strongly against Anastasia Pavlyuchenkova in the last round after weathering a two-set onslaught.
If she is to make her quarterfinal debut she will have to defuse the baseline bombs of another Russian, Ekaterina Makarova. Like Radwanska, Makarova is also looking to reach the quarterfinals for the first time, although she is unlikely to be overawed by the situation in front of her; Makarova is no stranger to an upset, with four Top 5 victims and a win over Serena Williams at last year's Australian Open to her name.
Around the grounds?
Preceding Radwanska and Makarova on Louis Armstrong is the day's other fourth round match, which pits the counterpunching of Angelique Kerber against the easy-on-the-eye game of Carla Suárez Navarro. Meanwhile, in the doubles several of the favorites are in action, including top seeds and defending champions Sara Errani and Roberta Vinci.