NEW YORK—The pro tour is built on the backs of players whose disciplined strokes produce a level of consistency that defines and separates the true professional from the amateur. Thus it’s unusual—and intriguing—to come across that special breed of player whose lack of consistency is a striking trait. You know, someone like Ernests Gulbis or, more to the point today, CoCo Vandeweghe.

Ranked No. 191, Vandeweghe slashed her way through three qualifying matches and the first round of the U.S. Open. But today the wheels fell off against—and this depends on how you look at it—either a woman she ought to have rolled through, or a deft, crafty lightweight who is expert at flummoxing straight-ahead power players like Vandeweghe. That was No. 18 seed Carla Suarez Navarro, who at 5’4” and 137 lbs. was giving away nine inches of height and a solid 20 pounds to Vandeweghe. Yet she eliminated the American with little trouble, 6-3, 6-4.

The match played out over an hour and 11 minutes on Court 17, a show court, with no great abundance of drama. Vandeweghe, whose forehand is a weapon and whose backhand certainly looks like it can bear a significant load, was broken early and often—the first time at 2-3 in the first set. It would be the only break Suarez Navarro needed to establish her lead.

Suarez Navarro suffered a scare early in the second set, when a determined Vandeweghe broke her for a 2-love lead. But the Spaniard broke right back, then jumped ahead when she broke again in the fifth game. To her credit, Vandeweghe leveled the set again with a break for 3-all, but she failed to hold the ground she’d reclaimed and gave Suarez Navarro the decisive break of the match and a 4-3 lead.

I watched this match and marveled at the parade of breaks with an old pal, and occasional contributor to TENNIS.com, Asad Raza. Neither of us could quite understand how little difference the vastly superior serve of Vandeweghe appeared to make. She routinely hit serves in the 120 M.P.H. range, while Suarez Navarro’s average first serve speed was 91 M.P.H.—just one M.P.H. faster than Vandeweghe's average second serve speed.

The statistics the told, indisputable truth: Vandeweghe may have dropped her serve so often because she put only 47 percent of her first deliveries into play. Yet there’s a caveat there, too. Vandeweghe has an excellent kick serve and doesn’t give up much having to fall back on it.

Here’s another killer stat that takes us to the intersection of Vandeweghe’s shortcomings: Suarez Navarro actually hit more unreturned serves—18 of 50, or 36 percent—than Vandeweghe, who was eight of 44 (18 percent) in that category. That’s a comment on Suarez Navarro’s returning proficiency, but it also suggests that Vandeweghe needs to work on her return game. The big difference between the two women was not serve conversion percentage or efficiency, but return consistency.

Now these are not written-in-stone tendencies. It’s the anatomy of one match, and let’s remember that Vandeweghe played five—and won four—here, just two fewer than the eventual finalists are likely to log. She put her loss down to the perils implicit in her game plan.

“I had to take chances in rallies, that’s probably why I made more unforced errors than she did (37 by Vandeweghe to 14 by Suarez Navarro),” she said. “I tried to take advantage of short balls and to keep her moving—that was the game plan, so it was unfortunate how many unforced errors there were.”

Ah, there we are, back at those “unforced errors,” which really gets back to this idea of consistency. With her atomic serve and persuasive groundstrokes, Vandeweghe could be expected to ruthlessly impose her game on a player like Suarez Navarro. That she was unable to do it ruined what was a good opportunity for her to keep pace with the Joneses better known as Stephens, Hampton, Keys, and McHale.

Vandeweghe professes not to be concerned with keeping pace with the emerging group of American women and is unfazed by the suggestion that she might be left behind. She told me, “Different players are doing different things, everyone goes at their own pace. You just got to keep driving through, week in, week out. I’m not like worried about other players, but thinking of myself and doing the right thing for me.”

So what is the “right thing” for someone who’s fast closing on 22 years of age but just plain hasn’t been consistent enough, often enough, to remain anywhere in the neighborhood of her career-high ranking of No. 69, a mark she hit a little more than a year ago?

The baffling thing to me is that, as a dedicated, successful, hard-working, aspiring pro since childhood, Vandeweghe has presumably hit as many balls as any of her peers. Why has her outcome been so different? Why do her sound and dangerous-looking strokes misfire so often? Is it some sort of mental shortcoming, some physical deficiency, or a combination of the two, in that all the physical orders executed by the legs and arm are first issued by the brain? In the end, why can some players routinely avoid thoughtless or sloppy errors, or string together 12 or 18 high-quality shots, while others can only accomplish that on rare, special days?

Vandeweghe believes the answer lies outside the realm of technique and/or temperament. “Today I felt like I was forcing some of the shots. I just should have played within myself a little more. I also need to keep improving my level of fitness. The better shape you’re in, the easier it is to make six or seven balls like you made the first three.”

She added, emphatically, “It’s physical for me, I know how to hit a ball and make it go in, at least sort of. . .”

That may be true, but that only leads to a different and no less obvious question: Shouldn’t a 21-year-old professional player’s fitness be a given? It’s not like Vandeweghe has been away from the game and is working her way back into shape; she qualified for and won a main-draw match at two of the big summer WTA events, Carlsbad and Stanford. (At the latter, she crushed No. 47 Monica Niculescu, 6-0, 6-3.)

I don’t pretend to have any answers, nor am I trying to run down Vandeweghe. But appears that she’s hit some kind of wall, and you have to wonder how she can clamber over it. This inability to hit an appropriate level of consistency is a problem that plagues many players, and one for which I’ve found too few satisfactory explanations.