Ryan Harrison, ranked 107th, was impressive Tuesday, dispatching former No. 1 Lleyton Hewitt in straight sets in the first round of the Citi Open. (Alex Brandon/AP)

Just 21, Ryan Harrison has a pretty mature understanding of what it takes to make it on the ATP tour.

“You’ve gotta love the process,” Harrison said after his 6-3, 7-5 win over former world No. 1 Lleyton Hewitt on Tuesday in a first-round match at the William H.G. FitzGerald Tennis Center in Northwest. “No matter how tense any moment can get out there, it’s what we live for, it’s what we play for.”

That mantra will be worth recalling Wednesday, when he will meet top-seeded Juan Martin del Potro, a Wimbledon semifinalist and two-time Citi Open champion, in the second round. Del Potro won’t be taking him lightly.

“He has a very good game, he serves very hard, he’s young, he’s trying to break into the top positions,” Del Potro said of Harrison. “He likes to play on hard court. He’s an American guy and I think the crowd will be cheering for him.”

When asked what song he’d enter the court to if given the choice, Harrison cited Toby Keith’s “Made in America” with little hesitation. Fitting for a player many believe could play a key role in the future of U.S. men’s tennis.

Harrison was born into a tennis family. His father, Pat, played collegiate tennis and works at the Bollettieri Tennis Academy in Bradenton, Fla. Ryan was born in Shreveport, La., and by age 11 he was already impressing on court, making it to the finals of the Shreveport City Championship. He lost to his father.

While Pat has coached Ryan in some form throughout, the Harrisons also hired private coaches to further his development. Most recently, Ryan had worked with Tres Davis, a former professional who Harrison first met at age 12 at a tournament in Chicago.

But after hitting a career-high ranking of No. 43 last July, his ranking has plummeted to 107 — prompting him to make a change and move to the U.S. Tennis Academy training center in Boca Raton, Fla., to work with USTA Coach Jay Berger.

“Tres and I are close friends, we’ve been involved, we still communicate about tennis, but it got to a situation where we had to reevaluate after the first six months of the year,” Harrison said. “He wants what’s best for my career, just like I want what’s best for my career and we kind of decided that being based in Boca and training there with the competitive crop of guys they have down here was going to be the best situation for me.”

The switch seems to have reinvigorated Harrison’s faith in “the process.” After a week in Boca Raton, Harrison made an impressive run in the ATP event in Atlanta last week, falling in the semifinals to No. 21-ranked Kevin Anderson — and he has followed up with the impressive win over Hewitt. But he’s not willing to chalk everything up to the switch so early on.

“You never know when [breakthroughs] are going to happen. I also was down 1-2, break point in the third set in Atlanta and those are just moments that can change here and there,” he said. “. . . I believe that the work that I put in that week and a half down there in Boca and the hard work I was able to put in certainly helped in my Atlanta run and in the win here today.”

Joining Harrison in the second round will be Sam Querrey, the top-ranked American in the field. Querrey, whose first-round match against Uzbekistan’s Denis Istomin turned into the match of the evening.

Querrey, ranked No. 22, dropped a sloppy first set, 6-3. But Querrey stormed back with a 6-3 win in the second set, setting up an epic third set that went to a tiebreaker before Querrey survived, 11-9.

Querrey wasn’t the only American male to fare well Tuesday, as 20-year-old Jack Sock, 25-year-old Tim Smyczek, and 26-year-old Alex Kuznetsov all played their way into the second round.

On the women’s side, 18-year-old Madison Keys joined Alison Riske as the only women to advance past the first round. There were nine Americans in the 32-player draw.

A familiar face for D.C.-area tennis fans pulled the consensus upset of the day, as former University of Virginia standout, Somdev Devvarman, 28, knocked off defending Citi Open champion, Alexandr Dogopolov. The 24-year-old Ukranian is the 25th-ranked player in the world. Devvarman has never climbed higher than 62nd and is currently ranked 129th.