Women's final is between friends Serena Williams and Caroline Wozniacki

Sunday's U.S. Open women's final will be anything but just between friends, even if it is between friends.

Serena Williams, the two-time defending champion and 17-time Grand Slam winner, and Caroline Wozniacki, still hunting that first Slam title, will battle for the ultimate tennis prize in America at Arthur Ashe Stadium.

In the highly competitive world of tennis, where opponents are more enemy than empathetic, Williams and Wozniacki have forged a friendship off the court, though they remain fiercely competitive on it.

In Williams' case, her friendship with Wozniacki has come late in her championship career. Williams is rather famously aloof from most players; her closest friendship was and is with older sister Venus. Wozniacki is the more outgoing of the two, earning the nickname "Sunshine" early in her career.

Off the court, the two are radiantly friendly, but on the court, they are foes.

Williams has dominated Wozniacki; she owns an 8-1 record against her. Wozniacki's lone victory was at Miami in 2012. Wozniacki lost to Williams in three-set matches in Montreal and Cincinnati this August as Williams was on the way to winning both titles.

Regardless of who wins Sunday, you can bet that Williams and Wozniacki will be together a lot next week in New York. Williams will have a show as part of Fashion Week, and Wozniacki says she will be there for her friend.

"Serena is a fun girl. She's so nice to hang out with," said Wozniacki, who was photographed earlier this year hanging out with Williams in Miami. "Always makes me laugh and makes everyone around her laugh. Definitely a very inspiring person to be around."

Williams was happy to see each other in opposite sides of the draw, meaning that they couldn't meet until the final.

"Of course we were like, 'That would be great if we could see each other in the final' because we both, you know, hadn't had the greatest Grand Slam year. We were just excited to be there."

The 2009 Open was where Wozniacki made her debut on the American tennis stage. She lost in the final to Kim Clijsters, and her run here raised expectations of an impressive Grand Slam career. Wozniacki has won 22 tournaments and reached No. 1 in the world. But she's never raised a Grand Slam trophy.

"Going out to the final back then against Kim, I knew that it was going to be really tough. I didn't know what to expect from my nerves. She had been out there before," Wozniacki said. "So this time it's going to be different. I hope that I'll have more experience. I hope that I'll go out there and just go for it. I have nothing to lose; I only have things to win."

Winning this Grand Slam, or any major, would lift a burden on Wozniacki. "I would love to have a Grand Slam under my belt," she said. "It would definitely have the media stop talking about my lack of a Grand Slam, so that would be nice."

As for facing her good friend Sunday, she said: "I'm pretty good about separating things on the court and off the court. The friendship, while we are on the court, is put aside. We are both competitors. She has to win, obviously. We will just be out there and fighting for every point. After the match, one of us is going to congratulate the other and we're going to be friends again."

When Williams suffered a pulmonary embolism in 2011, Wozniacki visited her in a hospital in Los Angeles. The friendship, already solid, has blossomed from there.

"It's definitely grown. We always have been friends, but it's definitely grown a lot," Williams said. "We love each other, but at the same time, we want to win. It's an interesting friendship and unique. I think it will last way past tennis."