Juan Martin Del Potro is steadily gaining the maturity that, before too long, could catapult him to a Grand Slam title. The 20-year-old from Argentina is probably unprepared to win the world’s biggest tennis tournaments in 2009, but there’s no question that his career is headed in the right direction.

Friday afternoon in Melbourne, Del Potro outlasted Luxembourg’s Gilles Muller in four high-quality sets to advance to the fourth round. The 6-7 (5), 7-5, 6-3, 7-5 triumph offered yet another indication that a promising professional is ripening into form.

In his first nine Grand Slam tournaments, Del Potro advanced past the second round only once, and that was when he reached the third round of the 2007 U.S. Open. Taking his lumps as a young boy in a man’s game, Del Potro had to walk the path of peril, that journey through necessary losses on the way up the tennis ladder. Like anyone else who has ever hit a tennis ball for a paycheck, Del Potro had to wonder just how many on-court defeats he’d have to endure before he’d be able to lift his ranking and increase his yearly income.

Last summer, that moment of discovery finally arrived.

Del Potro took the North American summer hardcourt season by storm, winning four straight tournaments and then advancing to the 2008 U.S. Open quarterfinals. When Andy Murray finally ushered him out of New York, Del Potro had compiled a 23-match winning streak, serving notice that his ascendancy was at hand. The next question for the tall and rangy performer was this: Could Juan Martin Del Potro pick up in 2009 where he left off in 2008?

Today’s match told the tennis community that this young buck isn’t about to regress–not necessarily because Del Potro won, but because of the way in which he prevailed over a dogged opponent.

This tussle in Hisense Arena proved to be a high-level exhibition of shotmaking and ballstriking. Muller’s lefty serve is hard to return because the Luxembourg native uses a low toss that prevents a receiver from gauging the flight of the serve. Muller also leans into the court, as any serve-and-volley player should, to gain an extra push on his many forays to the net. The combination of disguise and speed enabled the Muller serve to hold steady through nearly two full sets. Without being troubled on his own serve, Del Potro still lost the first set in a tiebreak and found himself in a 4-4 tie near the end of set two. Could Del Potro avoid a mental letdown and survive Muller’s charge?

He would… but not after receiving his one huge gift of the match.

Serving at 4-all in the second set, Del Potro wobbled, while Muller–sensing a chance to take complete control of the match–used aggressive tennis to earn a break point. Given Muller’s ability to hold all 10 of his service games up to that point, Del Potro could not concede a break on his own serve.

On this break point, Muller pounced on a short ball and hit a solid, high-percentage forehand to the ad court, which is Del Potro’s backhand corner. The Argentine scrambled to retrieve the shot with an off-pace backhand, but Muller was at the net for a relatively easy volley. The second set–and very likely, the match–flashed before Juan Martin’s very eyes. Without playing poorly, he was still about to find himself trailing by a set and a break.

There was just one thing that prevented such a scenario from turning into reality: Muller biffed the sitter volley.

Given a reprieve, Del Potro held for 5-4. But with Muller still serving from a tree, the prospect of a high-pressure second-set tiebreak still lingered in Del Potro’s mind. If the Argentine wanted to improve his chances of winning, he needed to break before reaching the stage of a set that ultimately amounts to a crapshoot. When Muller, serving at 5-6, gained a 40-15 lead, that crapshoot was just one point away.

Suddenly, shockingly, that tiebreak never came.

For the first time in the match, Muller cracked under fire. Pressing too much and appearing nervous, the European quickly and almost casually lost the edge that had taken him so far. After winning three points in a row, Del Potro converted his first break point by outworking Muller in an all-court rally. In four points, the feel of the match completely changed. Playing with the hard-earned wisdom that came from two solid years of Grand Slam losses, Juan Martin Del Potro had weathered the storm against a hot player, and lived to split two sets. No veteran–not even 20-year tour stalwart Fabrice Santoro–could have displayed so much poise in the heat of a high-stakes battle.

Finally enjoying the fruits of parity–compared to the two-set deficit he just barely avoided–Del Potro ripped winners from every place on the court in the third set. Running forehands and sharp-angled returns enabled an invigorated 20-year-old to make Muller look noticeably slow. With two breaks of serve, Del Potro won the third set, 6-3.

In set four, Muller gained something of a second wind, and battled on even terms through 10 games. In the eleventh game, Muller–serving at 5-all–had a game point for 6-5 and the assurance of a tiebreak at the very least.

Then, much like the second set, the noose seemed to tighten around Muller’s neck. The Luxembourger went for broke on a second serve and hit it into the middle of the net. At deuce, Muller had a backhand sail wide on him. Given a break point, Del Potro surprised Muller with a rush to the net and a pair of superb volleys. Playing far beyond his years and using the tactics of a 30-year-old, the No. 8 seed earned the right to close out the match on his own serve, which he promptly did–at 15–to move to the round of 16.

Juan Martin Del Potro watched an opponent play high-quality tennis for the better part of four sets, and responded by uncorking 62 winners against just 25 errors, a tremendous ratio that any elite player would covet. Still alive in Australia and still growing as a player, it seems to be just a matter of time before Juan Martin Del Potro wins the biggest prizes in his sport.