Harrison falls at US Open, but still working hard

"I've got to win one match at this level before I can consistently win at this level," Harrison said. "It's a process and it's going to take, unfortunately, a little more time than I want it to because these guys are all really good."

Now that Roddick is on his way out, 61st-ranked Harrison knows more people are looking to him and the other young Americans to fill the void. In 2010, he was a qualifier ranked in the 200s and became the first American male teenager to beat a top-20 opponent at any Grand Slam tournament since Roddick in 2001.

Since then, Harrison has steadily risen up the rankings but is still in search of his next breakthrough moment. This week's events — Roddick's retirement, his second-round loss — didn't change a thing in his mind.

"It's not like I'm going to work harder — because I already want it and I already work really hard," Harrison said.

Harrison's serve, often clocked above 130 mph, didn't have the same kick in this match, in part because del Potro is one of the best returners in the game and put pressure on the young American. Harrison fell down a break early in the first two sets. He got in only 51 percent of his first serves and finished with five double faults.

"Things tend to snowball whenever you're playing against a guy who makes you play that well," he said.