Serena buckled mentally, physically

This is the impossible. It doesn’t happen because it can’t happen. Now, it did happen. Serena Williams lost in the fourth round at Wimbledon on Monday, 6-2, 1-6, 6-4 to Sabine Lisicki.

It’s not just that Serena lost, which seems nearly impossible enough. The bigger issue is this:

For maybe the first time ever, Williams buckled, both mentally and physically. Both, at the same time.

“I wasn’t willing or able,’’ she said, “or probably didn’t even want to hold my serve today.’’

Didn’t want to?

“I was being sarcastic,’’ Williams said. “. . . I had a little hesitation, and that explains it. I don’t know. I definitely feel I tried . . . I definitely have to know going forward, if I want to be successful I’m never going to do it backing off.’’

For the first time since, seemingly forever, the last American standing at a major is not named Williams. Sloane Stephens, 20, advanced to the quarterfinals by beating another prospect, Monica Puig of Puerto Rico. Stephens was far better than she had been at any other time during the tournament.

The torch passing? Someday, maybe. But this isn’t it.

The only time I’ve seen Williams fold mentally was a few years ago against Kim Clijsters at the US Open, where she just could not believe she was on the verge of losing, and then starting screaming and threatening a line judge who had correctly called a foot fault on her.

Of course, Williams has had injuries before, too. But this physical problem appeared to be less pain than fitness.

So what can we make of this? What’s the big picture? It could be that at 31, Williams showed her age. Or maybe this is just the freakiest Wimbledon ever, with Roger Federer and Rafael Nadal losing early. Or maybe the pressure of history is getting to her, as Williams is still chasing Steffi Graf for greatest player ever.

Or, maybe it was just a one-time thing that happens.

I’m going with the last option, the one-time thing. Nadal had a clear injury, his knee, that could be a problem the rest of his career. Federer was starting to look old before he arrived. Those are bigger issues that could last.

Williams was on a run as dominant as any she has ever had. That run has been a little overblown, though, as the players she is dominating now are not nearly the quality of the ones she dominated 10 years ago.

But this was one day, one match.

Most of the match was defined by the look on Williams’ face. She came out in the first set looking entirely uninterested. It was lack of regard for Lisicki’s game. It’s understandable, I guess, as everyone – including me – thought that no one was even good enough to challenge Williams.

Serena apparently thought that, too.

But Lisicki has always had a lot of potential and a ton of power. She serves just as big as Williams and is the No. 23 seed.

Lisicki blew Williams off the court in the first set. Not only that, but also Lisicki played smart. We’ve seen examples in tennis lately where power players decide to swing as hard as humanly possible against superstars and just hope that more balls go in than go out.

That’s not what happened here. Lisicki was running Williams all over the court. When the court opened up, that’s when she would blast for the winner.

Let me just say this: I have been critical of women on tour for trying to outmuscle Williams, or to run her around. It always seemed impossible to do those things. You can’t outrun her or outpower her.

This is the part where I eat my words.

Lisicki did it.

Williams was making casual errors. After awhile, you should have seen the look on her face: It went from disinterested to scared, to . . .

Don’t mess with me.

Williams tanked the last game of the first set to keep from burning up too much energy in an effort to save the set. Then, in the second set, Williams rolled. She crushed Lisicki. Then, she went up 3-1 in the third.

Now, we’ve seen this so many times with Williams over the years. The only person who can beat her is her. At times, she’s disinterested. Usually, that’s during non-majors, so she goes ahead and loses.

In majors, she finds a moment, turns it on and takes control. That’s what was happening, until . . .

Her footwork started getting sloppy. Up 3-1 in the third, she tried a dropshot, instead of crushing the ball. Lisicki ran it down and pushed it past Williams for a winner.

That flustered Williams. She lost her serve. And now she was running out of gas. Lisicki kept powering away and broke Williams’ serve three times in a row. The last time, Williams missed an easy overhead, pushing it long instead of crushing it down.

Lisicki only had to hold serve to win.

“When you’re playing and have absolutely nothing to lose,’’ Williams said of Lisicki, “you can play with so much freedom, and so loose.’’

Actually, maybe that wasn’t a comment about Lisicki so much as herself.