NEW YORK—When you watch Caroline Wozniacki's game casually, you might thing she’s little more than a wallboard. The ball comes back, and comes back some more. And it is true that consistency and defense are her specialties; no matter how many times her father comes out and urges her to give her forehand a rip, she’s never going to blow anyone off the court.

But those who watch Wozniacki closely know that when she’s on her game, she’s more of a brain than a board, and more of an athlete than many people might think. For much of her fourth-round match against Maria Sharapova on Sunday, Wozniacki, a former No. 1 and U.S. Open finalist, was on her game again. In what she said was one of the most satisfying performances of her career, she beat Sharapova for the first time since 2011, 6-4, 2-6, 6-2.

Wozniacki was her old, subtly disruptive self in the early going. Contrary to everyone’s expectations, she was the one controlling the rallies, with placement, variety, and a little bit of chutzpah. She broke Sharapova for 2-0 by following a crosscourt backhand with a crosscourt forehand winner. She hit an ace and a service winner to hold for 3-0, and came up with two winning volleys to hold again for 4-1.

Wozniacki changed directions with the ball enough to be unpredictable, but not enough to be risky. You could even see find her limit and pull back. At 3-1, 30-30, with things going her way, Wozniacki stepped forward more  aggressively than normal and went for a big backhand down the line. It caught the tape. The shot looked rushed, un-Caro-esque. She seemed to know it, because with her next backhand, she sent a careful slice approach down the line and won the point at the net.

“I’m serving well, I’m running well, I’m staying aggressive when I have to, and I make the right decisions at the right moment, and I enjoy playing,” said Wozniacki, who nearly dropped out of the Top 20 this spring, when she asked what she’s been doing right of late. She hit 22 winners and was eight of 11 at net, respectable numbers for a so-called wallboard.


Caroline Yes

Caroline Yes

Yet to beat Sharapova today, Wozniacki still needed some help. There’s only so much an underpowered counterpuncher, even at her nuanced best, can do against a player who hits with Sharapova’s pace. From near the court, the physical disparity between the two players was remarkable. Sharapova is often praised for her mental toughness and disparaged for her lack of grace and athleticism. Up close, though, you can see how long and imposing she is when she stands at the baseline. While none of her shots will ever be likened to oil paintings, her contact and timing don’t suffer because of it.

Sharapova, as she has done so often, dug in during the second set and forced the match to go her way. Now she was the one moving her opponent and winning the long points. But she couldn’t keep it up in the third set. Impatience and errors crept back into the Sharapova game, as Wozniacki, after complaining about the amount of time Maria was taking between serves, seized the initiative again.

“I think I just stopped doing the things that helped me get advantage of the points in the second set,” said Sharapova, whose low-margin style suffered much more than Wozniacki’s safer approach on this windy day. “I hit a good shot and I allowed her to get back in the point instead of looking to come in.”

Asked what Wozniacki has improved recently, Sharapova said, “More consistency. Able to do it better. I think her strength has improved incredibly well.”

Was that enough praise for one day? Maybe. The next words out of Sharapova’s mouth were, “I think that’s the flood warning!” Alarms had begun to go off around the grounds as a storm neared Flushing Meadows. Then she laughed and said, “Darn it, if only I was out there a little longer.”

Not for the first time did Maria seem to use a post-loss presser as a therapy session—she was in a much happier mood after that moment. And not for the first time did this most intense of competitors impress reporters with her willingness to answer questions honestly, with a level head, and without excuses just a few minutes after taking a tough defeat.


Caroline Yes

Caroline Yes

But this day belonged to Wozniacki. Always a fighter, she grew more vocal early in the third. Maybe it helped: She won the point of the tournament to break serve for 3-1, tracking down three seemingly ungettable balls and eventually forcing a missed volley from Sharapova. Wozniacki told the crowd, which had cheered her all day, to get out of their seats and make some noise. The momentum from that moment carried her the rest of the way.

“It means a lot to me,” Wozniacki said of the win, which she said “ranks up there” with her best. “The season for me has been a little bit up and down, and it’s so nice to kind of start feeling like I’m playing the way I want to.”

Wozniacki didn’t answer a question about her ex-fiance, Rory McIlroy, but it’s obvious that her tennis has flourished since their relationship ended and she’s had more time to concentrate on her game—as well as do the legwork required to run the New York City marathon this fall. Today, with “Sweet Caroline” booming over the loudspeakers after her win, the New York audience was happy to see back in form. Could Caro match Rory and, at the same time get the old Grand Slam monkey off her back, at the Open? By knocking off Sharapova, she put herself in the quarters, in a wide open half of the draw. The only higher-seeded player there is Jelena Jankovic.

One major obstacle does remain, however, in the other half. But Wozniacki sounds as if she’s been planning for that.

“I told Serena I’m pretty tired of her,” Wozniacki joked today, “Twice you beat me in three sets. I said, ‘Can you just get out of my way?’”

Not likely. But today Caro showed that she can still push a very good opponent out of her way, and out of the draw. She also may have given her fellow tennis players a piece of ominous news. Watching her run Maria's shots down down, they have to consider the terrifying thought that training for a marathon really does work.