NEW YORK — Her career was older, at 20 years, than the age of the opponent across the net. That wasn’t an excuse; it was just a fact, and it was possibly an explanation for why Serena Williams one more time fell a match short in a Grand Slam. She was just a little dull in the U.S. Open final, missing a vital something against the flatiron strokes of the fearless new ascendant, ­19-year-old Bianca Andreescu.

For these past two decades, the most dominant shot in tennis has been the first serve of Williams, a 110-mph jolt that shortened matches and disheartened all comers. Sometimes she used it as a bludgeon and sometimes a crutch while winning 23 major titles. But it was not the biggest shot on the court Saturday at Arthur Ashe Stadium — that was the chain-flail forehand of Andreescu in a 6-3, 7-5 victory. Williams’s serve, disconcertingly, was not a weapon; it was a liability, an exasperating, net-slapping illustration that her 37-year-old body would not cooperate.

What became apparent, even as Williams fought with guttural screams not to go down by 6-1 in the second set, was that her comeback after a difficult childbirth over the past two years has been harder and more complicated than anyone realized, including her. In four Grand Slam finals now, Williams has gone down in eight straight sets against a variety of elastic-limbed younger opponents, unable to summon a crucial measure of voltage. Take nothing away from Andreescu, who is 8-0 against the top 10 this season, but the final was patently Williams’s worst performance of the tournament.

“I can play better. That’s the only solace I can take,” Williams said.

A Grand Slam demands seven peak-condition matches over two weeks — and the U.S. Open demands something more. It requires dealing with flashing stage lights in a succession of late-night matches, and fending off the whine of jets, the jabber of baby moguls drinking their Grey Goose in the loges and jeering of sirens, the constant ambient noise that leaches into the stadium and can wear out the nervous system.

Maybe it was all just a little too draining. For whatever reason, Williams made just 44 percent of her first serves — an incurable deficit against an opponent with strokes of such magnitude. Every time Williams missed a first serve, she had a two-thirds chance of losing the point against Andreescu’s lacerating returns. It was baffling — Williams had barely dropped her serve all tournament — but Saturday every service game was a siege. Furthermore, she made 33 unforced errors.

With a 2-year-old on her hip, tweaky knees, a spasming back and a 38th birthday coming up in a matter of weeks, Williams must now examine what she has to do to be better physically in a Grand Slam final. If there was something encouraging to take from this Open, it was that before the final she had looked her sharpest in nearly two years. She has been frank about her struggle to manage training, traveling and chasing records while raising her toddler, Olympia, the invisible undertow of mental and physical exhaustion of motherhood and the split heart that all mothers have who love their work.

“Being on the court is almost a little bit more relaxing than hanging out with a 2-year-old that’s dragging you everywhere,” she said this week.

Other mothers have won Grand Slams, of course: Margaret Court, Evonne Goolagong and Kim Clijsters. But none of them did it just weeks shy of their 38th birthday. All were age 31 or younger.

According to Christopher Minson, professor of human physiology at the University of Oregon, and a noted researcher on athletes and aging, just some of the physical issues Williams has been dealing with would include a decrease in her maximal ability to use oxygen (meaning her ability to perform at extreme heart rates) and a deterioration in strength and muscle function.

“Around the age of 40, we start seeing real reductions in strength,” he says. “Fast-twitch fibers tend to decrease in number and function, and that will affect things such as speed, getting to the ball, explosive power and the quick stops and turns you have to make in tennis.”

But perhaps the biggest telltale slippage in an older athlete — and this would seem to be huge in winning a two-week Grand Slam — is the need for longer recovery periods to get back to full strength and full speed. Minson says, “In these large tourneys, the longer each game goes, the more she will be broken down and her ability to recover will suffer.” Over the multiple days and multiple matches of the past two weeks, he observed, “She has been really digging herself into a hole.”

The biggest factor in recovery for an athlete? The one thing a mother doesn’t have as much of anymore: “Sleep,” Minson says. “If you’re not sleeping well, then you’re not going to recover. It’ll be a hurdle for everybody, but for an older athlete it’s all these little things that make it harder.”

The fact that Williams is still viable in Grand Slams is a marvel in and of itself. But that’s hardly satisfying for a player who is on the cusp of Court’s record of 24 Grand Slam titles but who can’t seem to get her best self to show up in a final.