Sloane Stephens hits a return during her third-round win over Victoria Azarenka at the U.S. Open. (Julian Finney/Getty Images)

Sloane Stephens is not a wide-eyed girl anymore, so when Victoria Azarenka started off with a 101 mph serve and then attempted a drop shot one stroke later, Stephens charged ahead to extinguish it.

The defending U.S. Open champion sprinted from the baseline and smacked a forehand cross-court winner well behind Azarenka, spinning around for a teeth-baring yell while pumping both fists as the final score — 6-3, 6-4 — flashed on the monitors at Arthur Ashe Stadium. Friday’s third-round match was her third win against Azarenka this year — a noted shift in a series that once belonged to the two-time Australian Open champion.

The first three times they played, in 2013, 2014 and 2015, Azarenka never lost more than five games to the American in a match. Now, Azarenka can’t seem to catch a break.

“It’s not about what she’s doing differently much. I mean, she’s playing probably more consistent, but I feel that I’m just not closing in,” the Belarusian said.

Stephens had a more concrete answer as to why their head-to-head series now stands at 3-3.

“The first couple times I played her, I was just like the young gun, just happy to be on the court with someone who was ranked No. 1 in the world and had won a Grand Slam already,” Stephens said. “The tables were turned.”

The inverse trajectories of their careers are underscored by their head-to-head results.

Azarenka’s heyday was earlier this decade; she won the Australian Open in 2013 and 2014 to become one of just three women, alongside Serena Williams and Kim Clijsters, to defend her title at a Grand Slam event since 2010. But the former world No. 1’s career derailed because of an ugly custody battle following the birth of her son, Leo, in late 2016, and Azarenka, ranked No. 79, is still finding her way back.

Stephens, by contrast, has grown up in the past year, and her game has matured.

She stunned the sport a year ago by winning her first Grand Slam after missing 11 months because of a foot injury, then she all but crashed from the whiplash. Sponsorships flooded in from Mercedes and Nike, and so did demands on her time; fatigue set in after that. Stephens, 25, lost her next eight matches after her U.S. Open win, including her first-round match at the Australian Open.

“There’s always a lot of extra things that happen after winning. An American winning the U.S. Open is pretty big,” Stephens said. “Definitely a lot of things to do. A lot less down days for myself. But that comes with the territory. I think I handled it the best that I could.”

Stephens, with Coach Kamau Murray and mother Sybil Smith by her side, leveled out in spring. She won her first WTA Premier Mandatory tournament, which is the level below a Grand Slam, in Miami and made the French Open final, where she lost to Simona Halep. In the run-up to the U.S. Open, she lost to Halep again in the final in Montreal.

Those big moments added up. She returned to New York to defend her title feeling — and playing — like something closer to a veteran.

“From last year, a lot has happened. I have played a lot of tournaments, been in really big moments, and I think just learning from every experience, being in another final, semifinal or quarterfinal. I think every tournament, when you find a good rhythm and something that works for you, you should just keep at it,” Stephens said.

Her movement was superb and her play from the backcourt was typically effective against Azarenka, 29, but the match was won on close points that could have gone either way. Stephens prevailed after a 19-shot rally to save a break point tied 1-1 in the second set then went up 3-1 before Azarenka surged back.

Even as her serve dropped — she missed just one first serve in the first set and 16 in the second — Stephens finished points better than Azarenka. Azarenka’s power and shot-making remain; it’s easy to see where lesser opponents would have let her back in the match. But Stephens knew how to handle the moment.

The crowd at Arthur Ashe Stadium rewarded her with a standing ovation. Stephens advances to face 22-year-old Elise Mertens of Belgium in the fourth round Sunday, with a possible matchup with one of the Williams sisters looming in the semifinals for the No. 3 seed.

She played Venus in the U.S. Open semifinals last year. She hasn’t played Serena — the favorite to win Friday night’s Williams-sister matchup — since 2015.

Serena beat her three times that year, but much has changed since then.

“A lot of things have happened in my life, my career, and what have you, but I think, yeah, I’m playing well now,” Stephens said. “I’m just happy with the progress I’ve made.”

Defending champ survives

Rafael Nadal, drenched in sweat and draped in a hot-pink tank top, stretched his arms out wide and tossed his head back in jubilation.

His match with Russian Karen Khachanov preceded the night’s main event between the Williams sisters, but it turned into one of the matches of the tournament, a 4-hour 23-minute battle at Arthur Ashe Stadium that felt like anything but an undercard. The defending champion staved off a gutsy upset effort to prevail, 5-7, 7-5, 7-6 (9-7), 7-6 (7-3), with the stadium roof closed for the final two sets.

The top seed advances to play Nikoloz Basilashvili in the fourth round.

Nadal was pushed to play his best tennis Friday by the 22-year-old from Moscow, and he responded. He was at his best in the third set, when Khachanov saved three set points in a tiebreaker. The Spaniard finally won it on a spectacular 39-shot rally.

“I needed that set,” Nadal said afterward.

The No. 1 player in the world wore tape just below his right kneecap for almost the entire match but didn’t appear hampered. He was his usual slick, flashy self, his movement unencumbered as he sprinted from one corner to another to dig out Khachanov’s groundstrokes, which seemed to find the edge of every line.

He was one point away from dropping the fourth set as well, but the three-time Grand Slam champion escaped again in a tiebreaker.