Sam Querrey wasn’t thinking about which of his fellow Americans were left in the tournament during his U.S. Open second-round match. Playing his best tennis of the year, Querrey was thinking only of his 6-3, 6-4, 6-4 upset of No. 28 Guillermo Garcia-Lopez .

Querrey’s win made him one of just three American men still alive in the singles draw. He joins No. 13 seed John Isner and Tim Smyczek — a trio that will have increased pressure going forward, given that the last American man to hoist the champion’s trophy here was Andy Roddick in 2003. That pressure is something they would prefer not to think about.

“At the end of the day, it’s an individual sport,” Querrey said. “I don’t really care too much. I mean, I want all the Americans to do well, but I want them to do well for them.”

Isner also won his second-round match, handling Jan-Lennard Struff, 7-6 (7-5), 6-4, 6-2. Smyczek’s second-round match will be Friday against No. 17 seed Roberto Bautista Agut.

Not long ago, the 26-year-old Querrey’s win would not have been considered an upset. With a big serve and powerful forehand, he advanced to the fourth round here in 2010 and was ranked as high as 17th in early 2011. But he struggled the rest of that year, fell out of the top 100 and has battled injuries since. He came to New York ranked 57th.

“Any seasons and tournaments and careers can sort of turn on the slightest, smallest things,” said Isner, a former doubles partner of Querrey’s. “Maybe he hasn’t had that much stuff sort of go his way, some bad luck, lost some tough matches and his confidence dipped a little bit, but it’s very encouraging to see him playing better now.

“You know, the way he played today, I saw a little bit of it. That’s the way that, when he was top 20 in the world, that’s the way he was playing. So he certainly has it in him, and it’s great to see him playing well.”

Querrey lost in the second round of Wimbledon and didn’t make it past the second round in his next three tournaments, but when he played in Winston-Salem, N.C., the week before the U.S. Open, he advanced to the semifinals.

Querrey leaned on his reliable serve against Garcia-Lopez, winning 88 percent of his first serve points and acing Garcia-Lopez 16 times, double faulting only once.

“I’ve got some confidence right now,” Querrey said. “I had momentum after last week, you know, putting together four wins. I’m just committing to hitting the forehand big and serving well, and it seems to be working.”

Querry’s countrymen didn’t fare so well through the first and second rounds. Donald Young, the second-highest ranked American entering the tournament, lost his first-round match. Steve Johnson and Jack Sock both had to retire from their first-round matches, and the slew of young Americans that received wild cards suffered expected early exits.

Querrey and Isner said it’s fair that questions about American men’s tennis and its shortcomings are more frequent during the U.S. Open, when tennis in America is spotlighted.

The answers range from young players saying the drought of a Grand Slam champion is motivation, to Isner bluntly saying the state of the sport in this country is not his concern.

After losing their men’s doubles match, Stefan Kozlov, 16, and Noah Rubin, 18, sat beside each other and pondered why there hasn’t been an American Grand Slam champion since Roddick in 2003.

“The United States is one of the greatest countries out there,” Kozlov said. “Since ’03? That’s ridiculous.”

“Kids aren’t given tennis rackets when they’re young,” Rubin explained.

“Other sports come first here, like football, basketball, golf,” Kozlov agreed.

It gets tougher for the Americans remaining. Isner will play German Philipp Kohlschreiber in the third round for the third year in a row; he lost to him the past two years.

Querrey, meanwhile, will meet top-seeded Novak Djokovic, who beat Paul-Henri Mathieu, 6-1, 6-3, 6-0.

“Sam is playing in front of his crowd,” Djokovic said. “I’m sure that he’s going to have some good support. . . . If he serves well, he’s very dangerous. I saw a little bit of his match today, and he played great. He’s in good form, good shape and has nothing to lose.”