SINGAPORE—Before they began their match, Maria Sharapova and Caroline Wozniacki likely figured that this wouldn’t be a quick day at the office. About seven weeks ago, Wozniacki outlasted Sharapova in the fourth round of the U.S. Open, 6-4, 2-6, 6-2, in two hours and 37 minutes. By the end, Wozniacki looked a whole lot faster.

That's also what occurred in Singapore: Sharapova worked incredibly hard to grab the second set, but Wozniacki skipped right out into the third. She danced back and forth and won it, 7-6 (4), 6-7 (5), 6-2, in three hours and 13 minutes.

Sharapova quickly walked out of center court after the loss and shook her head. Wozniacki smiled and said that it was a great match, even though she yelled and screamed during it because of lighting issues and a significant missed line call. She looked rattled after the tiebreak, but the world No. 8 has been more mature this season, a big reason for her presence in Singapore.

As Wozniacki said after the match, she wasn't just going down quietly. After all, she will compete in a 26-mile marathon in New York next month and feels spry.

“I kept thinking to myself out there in the third set, if you're going to get tired now, how are you going to get through this marathon?" Wozniacki said. “You better keep going. I did, and I felt pretty good out there. I felt I could still keep going for a while.”

The Danish 24-year-old finished 2010 and 2011 as the world No. 1, despite not winning a major title. But she began to slide in 2012, falling into a wilderness, and it has taken her nearly two years to become relevant again.

In August, Wozniacki began to be more aggressive and took more risks. She eventually reached the final of the U.S. Open, where she panicked a bit against her friend Serena Williams, but she kept moving forward.

Their rematch today showed just that. Sharapova had chances to overpower Wozniacki but made far too many errosrs, 76 in all. The Russian can hit a much bigger forehand than Wozniacki and go toe-to-toe with her backhand deep and into the corners, but she couldn't hit the lines time and time again. The same goes for her serve, which is bigger, but wasn’t accraute enough—she struck 15 double faults today.

Overall, Sharapova was pretty good, but not spectacular. Take the first tiebreaker, when Sharapova and Wozniacki were knotted at 4-4. From there she hit a big serve and missed an easy forehand, erred on a forehand sitting in the middle of the court, then double-faulted the set away.

Fortunately for her, Sharapova received a break of serve in the same way—via opponent’s errors—in the second set and was eventually able to even the match (the second set was well over an hour in length). But Wozniacki took an early break-of-serve lead over Sharapova in the third set, and this time, she didn’t panic. As Sharapova admitted afterward, she grew a bit sloppy and could not turn it around. The Russian (who essentially has lived in the U.S. for the past 20 years) agreed that Wozniacki has improved this year.

“She's always been one of the most physically challenging players to play against,” Sharapova said. “She gets many balls back and makes you hit an extra ball. I think she's doing that better. She's improved her strength in doing that.”

Sharapova will have to play much better if she wants to take out Petra Kvitova and Agnieszka Radwanska in hopes of reaching the semifinal.

“You can't dwell on it for too long. Sharapova said. “You've got to go out there and try to win the last two."