The downside to all this winning from Serena Williams

— -- NEW YORK -- Recently, Serena Williams admitted, it dawned on her that maybe there was a downside to this whole Grand Slam thing. That if she were to somehow make it through and win the US Open, she feared she would want to retire on the spot.

She was kidding, of course. She ended that story by saying she didn't want to retire because "I have a really cute outfit for the Australian Open [next January]."

But it's easy to understand where her mindset was. If we did not fully anticipate what a grind this campaign would be, the woman in the eye of this firestorm certainly did. And though surely she would like to avoid lurching through matches like her latest -- a 3-6, 7-5, 6-0 third-round victory over fellow American Bethanie Mattek-Sands -- this is what this journey has shaped up to be.

Looking understandably but utterly exhausted from a hard-hitting match and almost dazed by this singular experience, Williams reminded us Friday night it's all new to her.

"I don't know what to expect," she said wearily. "I've never been on this train. It's different. It's definitely different."

Against Mattek-Sands, it was also more of the same of what has plagued Williams throughout the year -- slow starts, three-setters and like her second-round victory here, some timing issues with her serve.

She talked about her high number of errors (28 unforced to 10 by Mattek-Sands). Williams also converted just 6 of 21 break points.

But she won the last eight games of the match and has now prevailed in 12 of 13 matches this year after dropping the first set.

Despite the shortcomings, there is the undeniable fact that sets her apart from all the rest.

"She's a great closer," said Mattek-Sands. "She always has been."

But while Mattek-Sands soaked up the "awesome" atmosphere on Arthur Ashe, Williams had the weight of a now 24-match US Open winning streak, a record 77-match wins in this cavernous stadium, including a record 30 at night.

That and the fact she is now four matches away from the first calendar-year Grand Slam in women's tennis in 27 years.

Though Williams said she was "not at all" nervous, there was still the matter of playing a worthy opponent. Mattek-Sands, who has won two doubles and one mixed Grand Slam title this year (four total in her career), has been ranked as high as No. 30 in singles (2011). She also has the sort of aggressive game and fighting spirit that was bound to give Williams some problems.

"I give myself a chance to win every single match I step on the court. Whether I win or not, I always think I can," Mattek-Sands said.

Next up for Williams is another confident American in Madison Keys, who is 10 years younger and 82 rankings spots higher than Mattek-Sands, but like the 30-year-old, Keys has little to lose.

And if Serena gets past Keys, her sister Venus may well be waiting in the quarterfinals with all the emotional baggage that comes along with it.

If anyone is expecting the pressure to suddenly be lifted off Williams' shoulders, it is certainly not her.

"I've never felt invincible," Serena said. "I've never used that word to describe me ever."

It is her experience and hers alone. And while it must be exhilarating, it must also be lonely. She does not know what to expect, and even if she were to ask Steffi Graf, the last to accomplish the season-Slam feat, it would likely not help Williams now.

Asked what it would take to derail her, she alluded back to her earlier metaphor and that unfamiliar ride on that unfamiliar train.

"If I'm standing on the middle of a track, I'm definitely going to get derailed," she said. "I have to make sure that I'm on the train and not, you know, in front of it."