Serena puts friendship aside against Wozniacki

‘What I really love about our relationship is we really play each other on the court incredibly hard. So, so, so, so, so hard. And then we just try to put it aside out of the court’

‘Quite frankly, I'm tired of losing to you’

There were no wild celebrations or fist pumps from Serena Williams after she beat Caroline Wozniacki at the WTA Finals on Saturday.

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Instead, the World No 1 was overcome with remorse after beating her good friend in an epic match that could have gone either way.

Williams, perhaps the most ferocious competitor women's tennis has ever seen, won 2-6, 6-3, 7-6(6) but her relief at winning was tempered by her feelings for her Danish opponent.

"Yeah, it was the first match I didn't feel great since obviously -- if you take Venus out of the equation -- where I wasn't happy after I won," Williams said.

"I didn't give a massive fist pump. I almost felt I really warmed to cry because Caro is so nice, we get along and we do so much together. It was really kind of difficult."

After spending years playing the same tournament, Williams and Wozniacki have struck up a genuine friendship, even though the 33-year-old American is nine years older.

When Wozniacki broke up with her fiance, Northern Irish golfer Rory McIlroy earlier this year, it was Williams who helped cheer her up, inviting the Dane to her Florida home.

They dined out together in Singapore before the WTA Finals began and on Friday -- the night before they were due to play each other for a place in the final -- they went to a Mariah Carey concert together.

"We've done it so many times this year it's becoming like clockwork. We hang out the night before and we have a match the next day," Williams said.

"It's just happening. We've played more times this year than we ever have in our careers. We both love Mariah Carey and we absolutely needed to go to the concert."

Last month, Williams and Wozniacki met in the final of the US Open. Williams won in straight sets, denying Wozniacki her first Grand Slam singles title, but the pair still partied together in Manhattan that night, with the American paying for their drinks.

Saturday's match was their 11th head to head meeting -- with Williams winning 10 and Wozniacki just once -- a statistic that Williams says she takes little pleasure from.

"What I really love about our relationship is we really play each other on the court incredibly hard. So, so, so, so, so hard. And then we just try to put it aside out of the court," she said.

"But I didn't feel great about it, to be honest. She's like my little baby sister from a different mother and father and different country."

For Wozniacki, the feeling is mutual. As much as she likes hanging out with Williams, she said she would prefer to see less of her on the court.

"I told Serena: Can you just leave me alone? Like, go away from the court," Wozniacki joked. "I love you, but stay away from the other side of the court. Quite frankly, I'm tired of losing to you."