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The future of U.S. men's tennis may hit new heights, but both the towering Sam Querrey and John Isner have a tall order in front of them to become truly elite players and be considered legitimate Grand Slam contenders.

After they stung the Memphis field and faced off in the final on Feb. 21 -- a 6-7 (3), 7-6 (5), 6-3 victory for Querrey -- both served notice that with monstrous serves and super-aggressive postures from the baseline, they have the weaponry to put out top-flight players. The 6-foot-9 Isner is now ranked No. 21 while Querrey sits at No. 22 -- both career highs. The top 20 is almost assured for both of them, but it's the more prestigious top 10 that they are aiming for.

"I believe 100 percent that Isner and Querrey have the ability to keep moving up the rankings," Tennis Channel analyst Justin Gimelstob told FOXSports.com. "They have exactly what you need: huge weapons with their serves and forehands, they play the modern game and both are difficult to match up against. They are both getting stronger, moving better and both have tremendous upward mobility in improving their games and skills. They are young and have the ability to have big tournaments and make big jumps."

Given how hard they serve and how nearly impossible it is to break them on fast surfaces, the 24-year-old Isner and 22-year-old Querrey can at least hang around matches until they find their return games, which are at this point, mediocre at best. Neither man reads his foes' serves particularly well, and due to an overall lack of speed, they don't win a lot of scramble points when their opponents dictate first.

But when they get their noses in front of points, they can be deadly. Just ask top American Andy Roddick, who has taken losses to both of them in the past six months and likes their long-term potential.

Do the two good friends have Roddick's unquenchable thirst for greatness? Querrey has said in the past that he admires Roddick's dogged work ethic and his willingness to scrape for every point. Roddick wins plenty of matches just by being the more mentally tough foe, something that Isner and Querrey can both learn from.

"They have a different fire, not as overt as Andy's," Gimelstob said. "Querrey lost a heartbreaker to Roddick in San Jose and I told him after that match that you'll have your time and the next week in Memphis he had his time and he beat Roddick. Under those laid back personas are fire and intensity that should not be underestimated."

Isner has proved that to some degrees at the majors, reaching the fourth round of the both the 2009 U.S. Open (where he upset Roddick) and the 2010 Australian Open (where he faced down Gael Monfils). He also won his first career title at the start of 2010 in Auckland.

Querrey has won three career titles -- in Las Vegas, Los Angeles and Memphis -- and has not only beaten Roddick, but owns wins over the elite likes of Nikolay Davydenko and Tommy Haas.

The modern game not only sports those two giant men but also has two top 10 players who are also skyscrapers: U.S. Open champ Juan Martin del Potro (who is 6-foot-6) and Aussie Open semifinalist Marin Cilic (6-foot-5). Both the Argentine and Croat move very well for their size and are rock solid from the backcourt, while both Isner and Querrey have trouble pasting winners off their backhand sides. But Gimelstob says the Americans are catching up.

"John has made big strides athletically and is trying to find the style to maximize his strengths," he said. "Querrey moves incredibly well and he's getting way stronger. Cilic and del Potro are immense talents, are technically sounder, their bases are a lot more solid and their games are further developed. But their margins to improve are a lot thinner than Querrey and Isner's."

U.S. Davis Cup captain Patrick McEnroe is hoping both men catch fire in the March 5-7 Davis Cup tie away in Serbia, where they will have to face a deep and talented Novak Djokovic-led squad on clay. For the first time since 2000, neither Roddick nor James Blake will be on the roster. Querrey will be making his second appearance for the U.S. team, while Isner will be making his debut.

Querrey went 0-2 in his Davis Cup debut against Spain in 2008, losing to Rafael Nadal on clay in his first match, but playing him fairly tough. McEnroe will certainly bank on the top ranked Bryan brothers to win the doubles point but isn't counting out his new guys pulling off singles wins. This week in Acapulco, Querrey pushed French Open semifinalist Fernando Gonzalez to a third-set tiebreaker, but Isner fell to the underwhelming Simon Gruel.

"Both guys have really stepped up their games and are more than ready to take on the challenge of going on the road to face Serbia," McEnroe said.

Gimelstob isn't dismissing their chances either.

"They are definitely underdogs and deservingly so," he said. "They will be playing in a hostile environment and the arena will pack fans in like sardines. We have a great doubles team, but Djokovic is a great clay court player and Janko Tipsarevic played well in Dubai this week, upsetting Andy Murray. We have to find two other points, but funny things happen in Davis Cup. The U.S. has a Buster Douglas type of chance."

If Isner and Querrey are to take the next step in their development, there's no doubt that they both have to improve their net games. Isner is a fair volleyer but has trouble with his transition game and low balls, which was evidenced in his loss to Andy Murray in Australia. Querrey grew up a power baseliner and has been slow to develop a love of the cords, although he can put away a passing attempt that comes into his wheelhouse. But at their heights and with their reach, there's no reason why they shouldn't be closing out dozens of points at the net. If they learn to do so, they could live up to Gimelstob's prediction that they are here to stay.

"Bridging the gap between the baseline and the net and adding that as a threat will be huge part of how far they move up," he said. "They belong in the top 20 and have an opportunity to pop into the top 10. There's no doubt in my mind these guys should be at top echelons of the game."