NEW YORK (AP) — Ryan Harrison is certainly OK with having U.S. tennis fans look at him as the country’s next potential star.

“Absolutely, I want to be that guy,” the 18-year-old Harrison said, before adding this note of caution: “I have a ways to go.”

If his latest match is any indication, he is on the right path. Harrison upset 15th-seeded Ivan Ljubicic of Croatia 6-3, 6-7 (4), 6-3, 6-4 in the first round of the U.S. Open on Wednesday, his first victory in the main draw of a Grand Slam tournament.

“The biggest win of my career,” said Harrison, who paused to sign autographs for fans in front-row seats before departing Court 11. “I’ve always believed in myself. I have always had confidence in myself, so obviously I’m extremely excited and really pleased with what happened.”

Then, as if catching himself sounding too excited about one win, Harrison quickly noted that he was eager to get “back into my routines of the day off, and looking forward to trying to get ready for second round.”

Already considered for some time to be one of the top young U.S. players, he’s now the first American male teenager to beat a top-20 opponent at any Grand Slam tournament since a 19-year-old Andy Roddick knocked off No. 11 Alex Corretja at the 2001 U.S. Open.

Before that, you need to go all the way back to 1991, when a 19-year-old Michael Chang beat No. 17 John McEnroe in New York — although by then, Chang already was a Grand Slam champion, having won the 1989 French Open at 17.

“I’m really putting all the work in. I’m trying to stay open-minded with everyone who is giving me their opinion and really trying to listen as much as possible and take in as much as advice as I can,” Harrison said.

One of the people who’s given him some tips: Roddick, who won the 2003 U.S. Open. That was the last time a man from the United States won a Grand Slam title.

Harrison, based in Bradenton, Fla., is ranked 220th and so had to qualify for the U.S. Open. But Wednesday’s victory is expected to allow him to crack the ATP’s top 200 for the first time; he would be the youngest current member of that group in the rankings.

“He’s definitely good; good player. I mean, he qualified here, which is, I think, great for him,” Ljubicic said. “You know, not just to get wildcards left and right whenever he needs and (instead), really try to struggle through the quallies. That definitely helped him for today’s match, as well.”

Harrison made his Grand Slam debut in January at the Australian Open, losing in the first round.

Ljubicic was a French Open semifinalist in 2006 and reached a career-best ranking of No. 3 that year. But he’s never been past the third round at Flushing Meadows and also lost in the first round in 2009.

Now Harrison will try to reach the third round by beating 36th-ranked Sergiy Stakhovsky of Ukraine, who advanced Wednesday by eliminating Peter Luczak of Australia 6-7 (8), 7-5, 6-4, 6-2.

“You always hear about … you have a big win and there is a lull match in there that you don’t play so well,” Harrison said. “The entire focus from the time I get back to the hotel tonight till when I play again on Friday is going to be preparation for the second round.”