To say that Jack Sock was a good junior tennis player is an understatement. He won national titles in every age group and a record number of USTA Gold Balls for victories.

Sock, now 21, earned exposure around the world in 2010, when he used a wildcard to get into the U.S. Open Junior Championships. He made his way through the draw to the winner’s circle, becoming the first American to win the event since his idol Andy Roddick took the title in 2000.

In July 2011, Sock officially turned professional—and he started his career with a respectable showing at the U.S. Open. He won his first singles match over Marc Gicquel before losing to Roddick, a fellow Nebraska native, in round two.

That wasn’t the end of the tournament for Sock, though. He and Melanie Oudin took advantage of a wildcard and won the U.S. Open Mixed Doubles title as an unseeded team.

Earlier this month, the ATP ranked him No. 79 in the world. His career has barely begun, yet Sock is already the third-ranked American behind John Isner and Sam Querrey.

Sock talked with STACK about his game and his latest off-court honor after his close (6-3, 6-7(1), 1-6, 2-6) third-round loss to No. 21 Janko Tipsarevic at this year’s U.S. Open.

STACK: When I saw you out there today, I was reminded that every time you go out there, you’re confident, whether you are playing the No. 21 player or the No. 300 player. How do you keep that up?

JACK SOCK: I think it comes a little bit from the juniors where I was staying in my own age division and playing a lot of guys who were supposed to beat me since they were top seeds in the draws. It kind of helps you with confidence going forward. I also feel I play my best tennis here in New York, so when I come out here, I have a chance against anyone.

STACK: What gets you going here?

JS: The excitement in the air. The fans are amazing. I don’t know what it is, but for some reason the courts and the balls are great.

STACK: You sustained some injuries in the juniors. In the pros, you’ve been healthy. What changed in your fitness and training regimen?

JS: In juniors, I had a stress fracture in my foot and then an abdominal [injury]. I’m getting in better shape. I feel great on the court. I feel I can go as long as I need to. I think that will pay off over time, especially in matches like today.

STACK: Is there anything specific in your fitness that you added or changed?

JS: I’m just doing a lot of running, weights, a lot of everything.

STACK: Have you changed anything in your diet?

JS: Maybe a little bit. I’m eating cleaner, but I think it’s more from the workout side of it.

STACK: You have a really big birthday this month.

JS: The big ’21’ [on Sept. 24].

STACK: Any plans?

JS: If I’m not playing, I don’t know. I’m not going to say it on tape.

[cf]skyword_tracking_tag[/cf]