NEW YORK—The first time Ryan Harrison saw Andy Roddick play tennis was at the U.S. Men’s Clay Court Championships at the Westside Tennis Club in Houston, Texas. Harrison was just eight years old at the time, and asked the rising American star to hit with him. Roddick declined.

These days, Harrison hits lots of balls with Roddick, who’s become a mentor to him, and says he expects to continue to do so even after the elder American hangs up his racquet following the U.S. Open. (The two live within miles of each other in Austin, Texas.) In competition, Harrison also hits balls with a great many players at Roddick’s level and higher.

Harrison’s problem isn’t hitting. It’s winning. In 14 at-bats against Top 10 opponents prior to this year’s U.S. Open, Harrison, currently ranked No. 61, hasn’t notched that elusive first victory.

The fifteenth Top 10 disappointment came this afternoon against Juan Martin del Potro at Louis Armstrong Stadium, where the Argentine won, 6-2, 6-3, 2-6, 6-2. The second-round encounter played out before a crowd that was at near capacity before the first ball was struck. American tennis fans are eager to coronate their next native champion and have had a perpetual eye on Harrison ever since he beat Ivan Ljubicic as a qualifier here in 2010, and then took Sergiy Stakhovsky to five sets in the second round in one of the more high-octane matches played on the Grandstand in the past few years.

Given the hero’s crescendo that greets Harrison whenever he steps on court here at the Open, it’s easy to forget how relatively meager his accomplishments to date are. At 20, this is just his ninth Grand Slam appearance, and he’s yet to reach any third round. He’s also yet to win a Tour-level title. Harrison openly and readily talks about his plus-sized ambition: After the match with del Potro, when asked about whether Roddick’s imminent retirement has lit a fire under young American players to step up, he replied that he couldn’t want it any more than he already does, that he couldn’t work any harder.

When he had his coming out party here in 2010, Harrison emerged as a ready-made star. His press conferences are thoughtful, if perhaps a bit practiced, and he oozes confidence and belief. It seemed somehow sad that he was relegated to one of the small, camera-less, stenographer-free interview rooms after his match today, where a mere handful of reporters sat around him like grade-school students. He seems to belong on a bigger stage, even in that off-court context.

From the seats in a tennis stadium, Harrison’s desire is also palpable, almost uncomfortably so. It often feels like he, with the help of well-meaning fans, has built a bit of a monster for himself: Treated like a star without having really earned it yet, there’s always pressure to get where he’s going, even if he’s not capable yet. He says that he knows he has a long way to go, and that the upside of going down to so many high seeds this year is that he has a lot of tapes to watch in the off-season and learn from.

To me, that was the most heartening thing he said in his press conference today, because we already know that he’s working hard. His tactics today, however, raise the question of what exactly he’s working on. On Wednesday night, before a packed house on the Grandstand, he comfortably handled Germany’s Benjamin Becker in straight sets. It was a confident, aggressive, dominating victory against a lower ranked opponent, and it was punctuated by fist pumps and screams to and from the crowd. But against a player of del Potro’s caliber, the flaws in Harrison’s game became manifest. The player who was so amped up on the other night turned in a surprisingly passive, defensive performance—underscored by a bite-less slice backhand—in which he rarely imposed himself and seemed to be waiting for errors that weren’t going to come, at least not often enough to make a difference.

The approach allowed del Potro to play much of the match with an energy and urgency that suggested a practice session. Harrison gave his forehands so much air and hit so many slice backhands that del Potro seemed utterly unhurried as he cocked his forehand and ambled from side to side, with all the time in the world to blast forehands to opposite corners of the court, running the American side to side—which he seemed all-too-willing to do. This, despite Harrison’s only hope of winning being to frustrate del Potro into an error, which happened a scant minority of the time.

One in a while, though, it worked, and it was concerning to me that Harrison seemed most energized when it did. It seems that nothing satisfies him more than showing how much he’s willing to hustle, and that when it works, he gets a natural high. The most extreme example of this almost masochistic tendency came on serve in the fourth set, when Harrison found himself down two break points. He saved the first with a cliffhanger rally that featured a drop shot, lob, smash, retrieved smash, and a running, down-the-line pass. It was a spectacular point, and he swung his fist in the air to celebrate as the crowd rewarded him with a collective roar. But with the possible exception of the retired Fabrice Santoro, this is not the material on which one founds a professional tennis career. It wasn’t even enough to get him out of the game, as he hit a forehand wide on the very next point, handing del Potro the break.

“Every match is a learning experience for me,” Harrison said afterward. He also admitted that when you play a man of del Potro’s ability, “things tend to snowball.”

He’s not the only one who looks up to a higher level of player. Even among the Top 10, tennis is stratified. In the press conference, del Potro was asked  if he and other players who hover just below the Top 4 suffer from a psychological disadvantage to Federer, Djokovic, Nadal, and Murray, one that allows that quartet to be the favorites (and winners) in major after major.

“No,” he replied. “It’s just the level. They are so good.”

Harrison’s right. It’s not about working harder. It’s about getting better, about getting as good as the guys at the top. Here’s to a productive off-season with those videos and gleaning the lessons contained therein.