John Isner has long been accustomed to carrying the torch for U.S. men's tennis, dealing with the heightened expectations at the Grand Slams and the inevitable questions that follow early losses.

Isner again faltered at the Australian Open on Wednesday with a defeat in the second round, but there was a bright spot for the Americans — a win by Jack Sock, the player now poised to potentially step into his shoes as the next U.S. men's hope.

It's been an unusually optimistic start to the Australian Open for the American men, with nine advancing to the second round — the most since 2008.

Numbers dwindled Wednesday, with Noah Rubin, Ryan Harrison and Steve Johnson following Isner out of the draw. But Sock and Sam Querrey both won, and three more Americans have a chance to join them in the third round on Thursday — Donald Young, Ernesto Escobedo and Frances Tiafoe.

Although there's considerable excitement about promising young players such as Tiafoe, Taylor Fritz, Reilly Opelka and Jared Donaldson, Sock is perhaps the closest to making a breakthrough on the big stage.

The 24-year-old Sock is coming off his best year on tour, winning two medals at the Rio de Janeiro Olympics (a gold in mixed doubles and bronze in men's doubles) and reaching his first Masters quarterfinal in Shanghai.

Sock started the new year by winning his second title in Auckland, New Zealand, and hitting a career-high ranking of No. 20.

Now, following his 6-3, 6-4, 6-4 win over Russia's Karen Khachanov on Wednesday, he's into the third round of the Australian Open for the first time. He next faces 12th-seeded Jo-Wilfried Tsonga, who beat him at last year's U.S. Open.

"I always have little landmark goals, I guess, and obviously top 20 was to get there, but my goals and aspirations are higher than that," he said. "Big goal for this year was to win more singles titles and I've got one already, so (I'm) trying to put one foot in front of the other."

Sock has long had one of the most fearsome forehands in the game — on par with Rafael Nadal and Juan Martin del Potro — but he spent the off-season improving his conditioning and now believes he's in the best shape of his career, as well.

"Confidence is high right now," he said. "I put in a great November and December of work and I think it's paying off."

Isner, meanwhile, was searching for answers after another early loss in Melbourne, where he's never advanced beyond the fourth round.

This one particularly stung: He blew a two-sets-to-none lead and wasted two match points, as well as 16 of 17 break points, before losing to Mischa Zverev 6-7 (4), 6-7 (4), 6-4, 7-6 (7), 9-7.

"I felt like I could have won the match in straight sets," he said. "I was just losing my mind out there a little bit, unfortunately. It's very disappointing."