NEW YORK (AP) - If there s a player out there who could possibly test No. 1-seeded Serena Williams in the U.S. Open final, it s her opponent on Sunday, No. 2 Victoria Azarenka.

Williams, it s worth noting, has lost only four of 70 matches this season, and two of those defeats came against Azarenka, including just last month at a hard-court tuneup.

There s one significant switch in circumstances: Those were at smaller tournaments. This time, there s a major championship on the line, and Williams is at her best when the stakes are the biggest.

"Different energy, different opportunities. This is for a Grand Slam," Williams said. "I mean, she s trying to win yet another one; I m trying to win one myself. It s just different."

Williams will be playing in her third consecutive U.S. Open final, seventh overall, and the 21st major title match of her career. Her win-loss record is 16-4, two Grand Slam titles away from two of the greats of the game: Martina Navratilova and Chris Evert, who each won 18. The only women with more are Margaret Court with 24, Steffi Graf with 22, and Helen Wills Moody with 19.

"Yeah, I thought about that," Williams said, "but I can t think about that. It s still so close, but it s still so far."

If there were any questions about what Williams considers the truest measure of success in tennis, she made clear how she feels when she was asked to pick between Roger Federer, who owns 17 major trophies, or Rafael Nadal, who has 12.

"I go by numbers," Williams responded. "I don t think I m the greatest, because Steffi has way more Grand Slams than me. I just go by what s written down."

Most of the numbers, from their careers and these two weeks, favor Williams over Azarenka.

They ve played each other 15 times in all, and Williams has won 12.

When they met in last year s U.S. Open final this will be the first rematch at Flushing Meadows since the Williams sisters played each other in 2001 and 2002 Azarenka pushed it to a third set and was two points away from victory. But Williams took the last four games.

"Not that I don t care what happened in the past, but I think there is no need for paying so much attention to what happened. (You re) always going to take a few things, positive or negative, and try to kind of apply it in your new match, new performance," Azarenka said. "But it s always a new story. I don t even think that it s going to be close to the same as it was last year."

Well, then, let s examine the 2013 U.S. Open.

Through six matches, Williams has won every set, losing a total of only 16 games. Only once in the last 25 years has a woman dropped fewer games through six full matches en route to a Grand Slam final (Mary Pierce dropped only 10 at the 1994 French Open, then lost the final.)

Azarenka has lost two sets and 40 games, including 13 in her fourth-round match alone. Her serving has been particularly shaky: 31 double-faults, 10 aces, 20 breaks. Williams, in contrast, has been broken only twice so far.

So how can anyone solve Williams  game right now?

"You ve got to fight," Azarenka said. "You know, you ve got to run, you ve got to grind, and you ve got to bite with your teeth for whatever opportunity you have."

It s clear these two have separated themselves from the rest of the pack, and not just because they re the top two in the rankings.

They re the only women to reach two Grand Slam finals this season Azarenka won her second Australian Open in January, and Williams won her second French Open in June.

Williams leads the WTA with a career-high-tying eight titles in 2013; Azarenka has three. Azarenka is a tour-leading 31-1 on hard courts.

"Azarenka is the closest player to Serena in terms of level," said Williams  coach, Patrick Mouratoglou. "Azarenka is a champion, and when it comes to the big matches, she s always lifting her level. Always. So you have to be ready."