INDIAN WELLS, CALIF.—These are the matches that try a promising young tennis player’s soul. Not to mention his mind, his nerves, his manners, and most of all his ability to keep himself from hauling off and smashing a ball out of the stadium at any moment.

Jack Sock, the promising young player in question, appeared ready to pass all of those tests on Stadium 2 here this afternoon. He was playing Ivo Karlovic, which is always a trying experience in itself. At 34, with a ranking of 143, the 6-foot-10 Dr. Ace isn’t the stuff of nightmares that he once was, but he can still take the racquet out of an opponent’s hand and leave him staring at the sky in blind frustration. To add irritation to annoyance, there was also a desert wind swirling through the stadium, which made Karlovic’s floating returns of serve knuckle and dance. Like so many players before him, Sock kept getting close to breaking the big man’s serve, only to watch in agony as a return of serve at break point would catch the tape and fall back, or Karlovic would reach out and stab a shank volley for a winner.

Sock couldn’t even count on the backing of his fellow Americans today. After losing the first set, Karlovic, hitting listlessly and seemingly out of options, decided to play the clown. After a winner of his was met with near silence, he threw his hands up and demanded that the mellow California crowd make some noise for him. After hitting a net-cord winner, he kissed his racquet and crossed himself. After losing a point on a net-cord winner by Sock, Karlovic pointed his finger to the sky and scolded God for taking the point from him. Finally, near the end of the second set, when he dug out a lucky volley on a break point and won the point on the next ball, he wagged his finger, Dikembe Mutumbo-style, in Sock’s direction. The audience loved it.

Through two sets, Sock didn’t appear fazed by any of this. The Nebraska native, who did his training in Kansas, knows a thing or two about playing tennis in a breeze—“I like the wind,” he said today. Sock sent the ball down the middle of the court and gave himself plenty of room when he went for winners with his forehand. By the time he reached the second-set tiebreaker, it was hard to remember the last time Sock had missed one. The beefy 20-year-old is the spitting image of a Nebraska farm kid—never mind that his dad is a financial adviser—and he was playing with the loose confidence of youth against a man whom he knew had seen better days. Sock said later that he felt like he “had the match in my hands.”

But with age comes experience, and few men have trudged through as many tiebreakers as Ivo Karlovic. Conversely, with youth comes youth. Sock had hardly missed a forehand all set; in fact, nearly every one he struck had seemed to go for a clean winner. At 0-1 in the tiebreaker, Sock went for another forehand...and missed it wide. There was no reason for it, except that this was a tiebreaker. Karlovic took that single mini-break and ran out to a 6-3 lead. Sock made a valiant comeback to 6-6 and even earned a match point. Once again, Karlovic came to the net, reached up for a lob with his long right arm, and stabbed at a shaky overhead. Sock, trapped in the corner, went for broke on a backhand pass and missed. Two points later, he went after another forehand, and missed again. The set was gone, and so was Sock’s self-control. He finally hauled off and sent a ball out of the stadium, deep into the desert. It might still be ascending as I write this.

It turned out that the match, and a second-round encounter with Davis Cup teammate Sam Querrey, was gone as well. Sock was broken in his first service game in the third set, and went away quickly after that. In the blink of a missed backhand, Sock went from looking like a confident gunslinger to looking like an unhappy former junior champion who had taken another punch to the gut from a veteran pro. That’s pretty much how he looked in his press conference afterward.

“He happened to win,” Sock concluded, when he was asked how the match had gotten away from him. “When you have match point, you seem comfortable out there. Pretty routine backhand up the line and I missed it by a couple of inches. Missed a simple forehand to lose the set.”

While his demeanor was downbeat, and he spent much of his presser with his head in his hand, Sock managed to take some positives away from his recent play.

“I’ve had a decent couple of weeks, Memphis through here,” Sock said, referencing his win over Milos Raonic two weeks ago, “just playing better tennis and happier with where I am.”

Indeed, for the last couple of weeks, it had seemed that, among young American men’s prospects, Sock was in the ascendancy and his fellow Davis Cup practice partner Ryan Harrison was in decline. How quickly things can turn around—or at least shift. Harrison played well in a three set win last night, and will have little to lose when he takes on Rafael Nadal tomorrow night. With a decent showing in that match, his stock could seem to be on the rise again.

All of which shows that there’s no use making any judgments about a young player’s future after one, or two, or even 10 results. For the long term, the important thing to know about Jack Sock is that he can hit a 100-m.p.h. forehand, and that he can hit it for winners from the baseline. Today he failed a couple of the other tests that turn a big-swinging prospect into a mature winner on tour. Sock failed to get that one return over the tape that would have broken Karlovic. He failed to hit the line with his backhand pass at match point. He failed to bounce back after a stinging tiebreaker defeat.

That’s not all that much—a matter, as Sock said, of a couple of inches. But those are the inches that try young player’s souls, and make them champions.