Serena Williams of the US poses with her trophy after winning the women's singles final match against Victoria Azarenka of Belarus at the US Open tennis tournament in New York's Central Park, Sept 9, 2013. [Photo/Agencies]

Williams looked full of life on the court in her two-week US Open run, overwhelming opponents on her way to the final.

She looked poised to make Azarenka her seventh successive straight-sets victim when she was serving for the match twice in the second set, before she admittedly "tightened up" and was forced to a tiebreaker won by the battling Belarussian.

Williams said that despite all her success, she has a tendency to get more nervous now than earlier in her triumphant Grand Slam career, which was launched with a victory at age 17 at the 1999 US Open.

"When you're playing for something different you start to think about it too much," Williams said, hinting at the history she is chasing with each additional Grand Slam win.

"That's what happened to me a little bit in the second set. I started to think way too much. But in the third, I just didn't care.

"I was just like 'this is what's going to happen, this is what I'm going to do and this is going to be the result,'" added Williams, who closed out the third set 6-1.

Williams said experience has taught her the importance of balancing two opposing elements required for her on-court success.

She said being "fierce" on the court was critical, but just as important was tempering that fiery emotion with composure.

"I'm better at being calm, more relaxed," said Williams, who still wears emotion on her sleeve in the glare of big matches, screaming at herself, gesturing and muttering in an interior dialogue to find the right mix of calm and fire.

Fitness at the relatively advanced age of nearly 32 is not an issue.

"I feel like jumping down and doing one-armed push-ups right now," the muscular Williams said, drawing laughs from the reporters. "But I can't, by the way."

Serena Williams repeats as US Open champion