US tennis players are impressing at the Australian Open: Four are already in the 4th round

It's been almost two full decades since this many U.S. men reached Week 2 at the Australian Open.

And while that group in 2004 included a couple of Grand Slam champions in Andre Agassi and Andy Roddick, everything is all so new for the quartet there this time: Tommy Paul, who is 25; J.J. Wolf, 24; Sebastian Korda, 22; and Ben Shelton, 20, are all about to make their fourth-round debuts at Melbourne Park.

It must feel very much like a chance for a career-defining result for them and other young men still in the bracket.

"I haven't thought about it too much, honestly, because I just have that one-match-at-a-time mentality, but I think it's hard for anyone to look past that. There's been a lot of upsets," the 67th-ranked Wolf, who played college tennis at Ohio State, said after eliminating lucky loser Michael Mmoh 6-4, 6-1, 6-2 in an all-American matchup. "But upsets happen for a reason. A lot of people out here are good. It is a real opportunity."

Wolf next plays yet another American, 89th-ranked Shelton, who won the NCAA title for the University of Florida as a sophomore last year, then turned pro. Using his passport to travel outside of the United States for the first time in his life, Shelton extended his stay in his Australian Open debut by defeating 113th-ranked Australian wild-card entry Alexei Popyrin 6-3, 7-6 (4), 6-4.