The 31-year-old has arguably never been more dominant.
A clear world number one, she has already won more matches and equalled her career-best haul of titles for a calendar year, including an unbeaten streak of 34 matches.
But two of her four defeats this year have been at grand slams, shock losses to Sloane Stephens in Australia and Sabine Lisicki at Wimbledon.
An ankle injury was mitigation in the Stephens defeat but against Lisicki a nervous Williams was simply beaten by the more secure player.
The American used to win grand slams even when she was not winning, and often not playing, regular tour events, but this year it has been the other way around.
She won the title she wanted more than any other, a second French Open crown, but if she fails to hold onto her US Open trophy then what appears a great year will need an asterisk.
Williams said: "I live to play the grand slams. Obviously I would love to trade those (titles) in, but you can't.
"I'm really proud of all the tournaments that I have won. I think ultimately a lot of the tournaments I won is the reason I was able to win the French Open."
Williams, who plays former French Open champion Francesca Schiavone in the first round at Flushing Meadows, is certainly the big title favourite, and any apprehension stemming from her grand slam losses is tempered by what she has already achieved.
The 31-year-old is bidding to match Roger Federer's haul of 17 slam singles titles, which would put her only one behind Martina Navratilova and Chris Evert.
"I don't need to do anything," said Williams. "That's the beauty of my career. I don't need to do anything at all.
"Everything I do from this day forward is a bonus. Actually from yesterday. It doesn't matter. Everything for me is just extra."
Williams' biggest rival for the title is expected to be second seed Victoria Azarenka, who is responsible for two of the world number one's defeats this year.
Unlike the absent Maria Sharapova, who Williams has beaten 13 times in a row, Azarenka has proved a good match for the American of late.
The Belarusian served for the match in the final here last year before Williams edged her out but Azarenka has won two of their four meetings since.
And the 24-year-old has the psychological advantage of having beaten Williams the last time they played, winning a deciding tie-break in the final in Cincinnati last weekend.
Williams thinks that result can boost her US Open cause, saying: "I'm definitely really excited.
"Every time I lose, I get so pumped afterwards and I just feel that way. I just feel like now I'm ready, now I'm prepared. I almost needed that to take my game to a new level."
Of her rivalry with Azarenka, she added: "I have had so many great rivalries. You never really know how deep a rivalry is until you play the person several times.
"It's just always good to play such great players like Victoria who really plays well and makes me lift my game to a new level."
Williams, meanwhile, insists her busy season is not taking its toll physically, in fact quite the opposite.
"I feel great," she said. "I feel completely recharged. To play more matches now later in my career than sooner, it's interesting how good I feel."