Caroline Wozniacki never doubted she'd find top form again

— -- NEW YORK -- At heart, Caroline Wozniacki is a marathoner. The two-time US Open finalist was one long before she ran the 2014 New York City marathon on a lark and finished in an outstanding 3:27. The 26-year-old Dane runs, runs, then runs some more, retrieving and returning shots most of her rivals in tennis can only watch fly by.

Madison Keys, a gifted 21-year-old, is a different breed of player and athlete. She has more of a sprinter's game, based on short bursts of brilliant shot-making with interludes of ordinary and sometimes even poor play.

Here at the US Open, No. 8 Keys had tiptoed along the edge of disaster, rebounding from threatening deficits in two of her three previous matches. On Sunday, Wozniacki jogged up alongside Keys and shoved her into the abyss, winning their fourth-round clash in just an hour and 18 minutes, 6-3, 6-4.

Count this one as another major step in Wozniacki's comeback from an ankle injury that helped drive her ranking down to its current No. 74. Wozniacki is taking her comeback one step at a time. Like a veteran distance runner, she isn't craning her neck, trying to spot the finish line. She knows that a full-on renaissance calls for stamina and patience.

"To be honest, at this point, I'm very much like I don't care [what my ranking is]," the beaming winner said after she won. "Because if I'm not in the top five, I feel like it's not where I want to be. So at that point, whether I'm 20 in the world or 100 in the world, it doesn't matter. I'm going to play the same people anyway."

She also professed that she has self-belief, and said it's all that matters, boldly adding, "I think it just sucks for some of the other players who have to play me early."

A rueful Keys agreed. Surprisingly, this was their first meeting, and it worked in Wozniacki's favor: "It's different playing and seeing it on TV," Keys said. "[I was] feeling how deep she's hitting the ball and feeling that I can't get on the offense right away."

This was the fifth consecutive major championship in which Keys, who is often touted as the heir apparent to Venus?and Serena Williams, faltered in the fourth round. But unlike a few of those losses, this was neither shocking nor complicated.

Wozniacki was the year-end No. 1 for two consecutive years, starting in 2010. She is fourth among active players with 23 singles titles and owns the dubious record of having the most career singles titles without having won a Grand Slam event. But she's still just 26.

The ankle injury that interrupted Wozniacki's career this spring (and caused her to miss a Grand Slam tournament for the first time) provided her with an unplanned opportunity to hit the reset button. Wozniacki has a reputation as the fun-loving BFF of Serena Williams, but as Wozniacki's marathon adventure suggests, she's always been driven. The recent break left her feeling a little more mellow.

"Before it was all about, 'OK, I have to win this; I have to do that,'" Wozniacki said. "Now I'm like, I don't have to do anything. You know, whatever is meant to be it's going to be, and I'm just going to give myself the best shot out there. And I think I'm enjoying the whole atmosphere and everything else so much more."

While reveling in the atmosphere, Wozniacki also provided the young American with a multidimensional lesson with one working premise: Be aggressive but keep the ball in the court. Keys has Part A down pat; it's the consistency that flummoxes her.

"It was a very fine line between being patient, consistent and also trying to be aggressive and not step too far behind the baseline," Wozniacki said of her strategy. "My main thing was to just go for my shots. Don't hold back."

Keys had just one break point. Her unforced errors exceeded Wozniacki's by multiple of four-plus (33-7). Keys' serve is already one of the most feared weapons in tennis; it rivals Serena' own. But here's the key stat: Wozniacki put 81 percent of her serve returns into play (57 of 70). Meanwhile, Keys managed to return just 66 percent (31 of 47) of Wozniacki's deliveries.

True, Wozniacki's serve is improved. But the theory that all Keys still needs to do to break through is more Grand Slam experience and a greater ability to win when she doesn't have her A-game is wearing thin.

The one thing all great players bring to the table almost every day is a baseline level of consistency. Wozniacki still hasn't won a major, but she has that fundamental ability to keep the ball between the stripes. She may never have the firepower of a Keys or even a Petra Kvitova, but Woz is young enough to win a major. She may finally win one next Sunday if she can continue to successfully marry consistency and aggression.

It's tempting to think that Wozniacki's biggest opportunities have passed her by. But this is a woman who can run and run. She's a marathoner. She may catch up with them yet.