There is just one thing left for Marcos Baghdatis to do at the Australian Open: beat Roger Federer.

And why not? Considering the surprises this 20-year-old from Cyprus has given us so far, nothing seems implausible. He downed Andy Roddick in four sets, dismissed Ivan Ljubicic in five, and topped both of those performances on Thursday night with a stirring comeback against David Nalbandian, an even bigger favorite whose steady, versatile game had been impregnable in his last two matches.

Baghdatis survived a two-set deficit, two breaks in the fifth set, the booming fireworks of Australia Day,a brief downpour in the final game, a 22-minute delay as the stadium roof was closed, and a questionable overrule on his first match point before prevailing, 3-6, 5-7, 6-3, 6-4, 6-4.

If these obstacles had been thrown at Nicolas Kiefer, who this week tossed his racket in the direction of an opponent during a point, security guards would have had to escort the volatile German from the court in handcuffs. Not Baghdatis. For the past two weeks, the man has redefined composure during an otherworldly run to his first Grand Slam final.

That this match reached a fifth set was something of a miracle. Nalbandian started slowly in this tournament as he recovered from a stomach ailment, but by the time he arrived at this semifinal, he looked every bit the man who toppled Federer in the final of last year’s Masters Cup.There is little that Nalbandian cannot do well. His judgment and court sense are sound, his two-handed backhand is one of the best on tour, and his forehand, though at times erratic, is a weapon in its own right. Little is said of his volleys, which are sturdy and put to judicious use: Before last night, he had approached the net 196 times, winning 149 (76%). Nalbandian also owns one of the best service returns in the game, something Baghdatis and his suspect second serve did not have to contend with against Roddick or Ljubicic.

On paper, Nalbandian was more than Baghdatis could handle,and the first 15 games played true to form. Baghdatis trailed 6-3, 5-1 and lacked the explosive movement and passing shots that had so rattled his previous opponents. After a fine surge of four consecutive games failed to win Baghdatis the second set, Nalbandian was a cinch for his second Grand Slam final.

One can only admire Baghdatis’s persistence. He stayed even with Nalbandian for a few games in the third set, then broke to take his first lead. His legs, undoubtedly tired from his two previous upsets,came to life.He played more aggressively as Nalbandian slowly tensed up. His running forehand, author of so many remarkable crosscourt winners these two weeks, was in full flow. Perhaps most important, he hit his first serve consistently for the first time in the tournament. In the second and third sets, Baghdatis made 60% of his first serves – he is usually closer to 50% – and won 91% of those points. Nalbandian had a look at 12 break points in the first two sets. He saw none in the third and three in the fourth, and did not convert any.

Nalbandian made obvious mistakes in the latter half of the match,missing ordinary forehands and striking a few important backhands with little authority. He could not have played worse than he did in the fifth set.Even considering that decisive sets are often sloppy affairs between exhausted players, Nalbandian’s performance, in a Grand Slam semifinal, was inexcusable. Baghdatis won 27 points in the fifth set, 15 of them on Nalbandian errors. At 4-4 in the fifth set, Nalbandian forfeited a love break on a double fault and three errors.

Nalbandian was also hurt by a subtle tactical failure. He did not recognize, despite repeated reminders in the form of scorched tennis balls, that Baghdatis loves to hit backhand service returns in the ad court, whether they be ropes down the line or crisp crosscourt angles. Nalbandian is not blessed with a powerful serve, and he compensates with precision and a mix of speeds and spins.One serve he seems to admire in particular, based on data from this tournament, is a kick serve, on his first serve, to his opponent’s backhand in the ad court. It’s the equivalent of a curveball on a fastball count, and quite often an effective play against an unsuspecting opponent or one with a weak backhand.

Baghdatis is not one of those opponents.Nalbandian hit the majority of his ad-court first serves, 24 in all, to Baghdatis’s backhand, many of the kick variety (his average speed on those serves was 95.7 mph, almost 15 mph slower than his serves up the middle). Baghdatis stepped forward and punished these offerings, winning 13 of them.

One proved crucial. At 1-1, 30-40 in the fourth set,Baghdatis crushed a crosscourt backhand return and lured Nalbandian into hitting a backhand down the line. Baghdatis unleashed one of his running crosscourt forehands, forcing an error. Overall, Nalbandian won 71% of the first serves he put in play in the deuce court, compared to 55% in the ad court. Baghdatis was much better, and when he hit wide first serves into Nalbandian’s strong backhand, he hit them with much more pace (an average of 119.3 mph).

Maybe we should not be surprised that Nalbandian underestimated one of Baghdatis’s strengths. Those same oversights have been made by all of us during this Australian Open, which, at the outset, promised to be a ho-hum sequel to last year’s fabulous centennial edition. Several stars were injured, Federer was dominant, and maybe Roddick or Nalbandian or Ljubicic had a chance. Then along came Baghdatis, fleet of foot and armed with compact, explosive strokes and a winning smile. He has single-handedly transformed this tournament into a thriller.

tperrotta@nysun.com