ESPN 2008 US Open

NEW YORK -- Andy Murray has taken to playfully flexing his biceps on court to make the point that he has added bulk to his formerly scrawny frame. Sunday, he muscled his way into an elite fraternity.

The big three of men's tennis -- Rafael Nadal, Roger Federer and Novak Djokovic -- officially expanded to become the big four as a young Scot long heralded as a future Grand Slam winner put himself in position to win his first. Murray outplayed world No. 1 Nadal in their rain-interrupted U.S. Open semifinal, winning 6-2, 7-6 (5), 4-6, 6-4 to earn the right to face four-time defending champion Federer.

Welcome to the club. To paraphrase a certain credit card company, membership has its privileges -- and pressures.

Federer dearly wants to win this tournament, silence the doubters, and verify that he's back on track and capable of fighting to regain the No. 1 ranking. A title here would put him one short of Pete Sampras' record of 14 Slams going into next season.

On the other side of the court, 21-year-old, sixth-seeded Murray will carry the fervent hopes and longing of a nation that hasn't had a men's Slam champion since Fred Perry in 1936. He's not new to the position, having had to deal with that spotlight since he was a rawboned, quick-tempered teenager -- although the chief burden at that time was borne by Gentleman Tim Henman.

Murray is fully aware that much of Great Britain will lose sleep over his every move Monday, but he has learned that if he feels fully prepared, the rest follows.

"I started to train physically way harder, you know," soft-spoken Murray said. "The pain that you feel off the court … when you're running around the track is much worse than anything you feel on the tennis court. I go on the court now without feeling like I have anything to worry about because I've worked hard and practiced hard and given myself the best opportunity to play well. All I've got to do is play tennis, which is one of the few things that I'm good at."

Murray, who always opts for the considered rather than the pat answer, said the pressures of being his country's standard-bearer weren't nearly as tough as the isolation and homesickness he endured when he went to Spain alone at age 15 to try to refine his obvious talent.

"When you go on the court and you haven't put in the work off it and you haven't practiced as hard as you should have done, you can find excuses for why you're not playing well or why you're getting tired and stuff," he said.

The final will pit two of the game's great thinkers and great defenders, who both possess wonderful touch, almost unparalleled versatility and the ability to turn coal to diamonds during a rally. But one of Murray's former coaches, American Brad Gilbert, said the match will boil down to a simple and familiar formula: "Whoever serves best will win."

Monday "matches up great for Andy," Gilbert said from his Bay Area home, where he was on the practice court with his son Zach, who plays at the University of California. "He's six years younger [than Federer]; he's incredibly fresh; and he's on his best surface, where he's beaten Federer the last two times they've played. He's got to feel good about his chances."

Federer had expressed a mild preference to play Nadal in the final, ostensibly because of their great rivalry, which seems to carve words into the granite tablets of the sport with every new chapter. But he signaled his competitive respect for Murray while the Nadal-Murray match was still going on.

"It will be interesting to see how [Murray] handles it, but I have a feeling he's a guy who plays well on the big occasions," Federer said, referring to their potential matchup. "That's why I need to be very, very careful how I play [Monday] if it is Andy."

Experience is an enormous plus for Federer, but there is the unvarnished fact that Murray owns a 2-1 career record against him. Murray was the only man other than Nadal to beat the Swiss maestro in Federer's brilliant 2006 season and topped him most recently in Dubai, a win that prompted an unsolicited critique from Federer.

"I don't think he's changed his game a whole lot since I played him in the Bangkok final [in 2005]," Federer said after the match in Dubai in March. "I really thought he would have changed it in some ways. It sort of works for him, but he's going to have to grind very hard for the next few years if he keeps playing this way. … He stands way far behind on the court, and that means you have to do a lot of running. He tends to wait a lot for an opponent's mistake."

Murray certainly ran his share of miles against durable Nadal, who is finally showing the wear and tear of his sensational season. "Too many matches in my shoulders," Nadal said afterward, wearing the sadly satisfied smile of a player who knows he did his best.

But Murray, now a strong, rangy 6-foot-3, made it evident that he has built up considerable mental muscle, as well.

Questionable scheduling decisions put Murray in one kind of hole. He was up two sets on Nadal when his momentum was literally dampened by the monsoon that rolled into the New York area Saturday afternoon. That match might have been completed had it started at the same time as the Federer-Djokovic encounter.

And Murray was able to recover from the psychological blow of first losing the third set when play resumed Sunday afternoon, then failing to convert seven break points in the second game of the fourth set despite throwing everything in his toolbox at the persistent Spaniard.

Sunday's play began not only on a whole new day but in a completely different microclimate -- cloudless and dry as opposed to the sticky conditions 24 hours before, a shift that might have seemed to favor the flight of Nadal's brawny groundstrokes.

Murray chose to fight that fire with fire-extinguishing changes of pace at times, a strategy that worked well for him in Part I of the match Saturday. He also equaled Nadal's punch by driving his lashing, precise backhand into both corners for winners, and in the final set executed one no-look passing backhand winner with a spin move worthy of Magic Johnson. But Murray's fluid movement might have been the difference in the match.

The Scot looked downcast and rushed through his own service game after his seven lost opportunities to give Nadal a quick break. But he pulled out of that tailspin to even things at three games apiece, earning a break point when Nadal -- perhaps trying to angle a volley a little too much when he saw Murray retrieve a shot, then dart back toward the center line in almost the same movement -- put the ball in the net instead.

On the penultimate point of the match, Murray kept Nadal on the move so long that the Spanish player bent over at the hips after swatting a backhand into the net, his hands clamped to his thighs in the athlete's universal sign of being gassed.

It was Murray's first win against Nadal in six tries, and his success in this tournament will elevate him to No. 4 in the ATP rankings next week.

Murray has proved he's of age, qualified to be in the big four club, and the bouncers won't have to check his identification next time he arrives at this point. Now it's a matter of performing on one of tennis' most brightly lit dance floors.

"I played well enough to beat the No. 1 player in the world over two days, and I've beaten Roger in the past," he said. "I have the tennis to compete with those guys. I just have to make sure I do it for three out of five sets rather than for a set and a half, two sets."

Bonnie D. Ford covers tennis and Olympic sports for ESPN.com. She can be reached at bonniedford@aol.com.