Need proof that it doesn’t take long to reverse one’s fortunes in tennis? Look at Andy Roddick and Gilles Muller.

Last month, Roddick played dreadfully at two small tournaments packed with second-tier players because the best men in the game were busy battling for Olympic gold. An injury to his shoulder had limited his time on the practice courts and his confidence was low. Who would have expected that Roddick would have a realistic chance at the U.S. Open semifinals and even the final?

At least Roddick was struggling on the pro tour. Muller, you might recall, wrecked Roddick’s 2005 U.S. Open, and the infamous American Express “mojo” marketing campaign, when he bounced Roddick out of the tournament in the first round. Since then the 25-year-old lefty from Luxembourg, a former U.S. Open junior champion who serves and volleys, has done next to nothing. This summer, he couldn’t win more than two consecutive matches at tiny tournaments in Istanbul and Segovia, Spain, where the prize money barely covers travel and expenses. In the qualifying event for this tournament, Muller nearly lost in the first round. Since then, he has won consecutive five-set matches — after losing the first two sets in each. Today, Muller will play Roger Federer in his first major quarterfinal. Win or lose, he’ll at least halve his world ranking, which stood at 130 when this tournament began.

Roddick ought to be thrilled by the way this tournament has shaped up. After struggling early in his second-round match, Roddick has tuned up two straight opponents. Meanwhile, his opponent in tonight’s quarterfinal, Novak Djokovic, has spent nearly four hours on court in each of his last two matches. He has rolled both his ankles. He has hurt his hip, nursed a stomach ache, and spent a lot of time hunched over and gasping for air. Djokovic’s last opponent, Tommy Robredo, suggested that Djokovic’s injuries are part reality, part act, an accusation Djokovic has heard before.

“I had pain as well, all over my body, because I think I run a lot more than him, and I said nothing,” Robredo said. “I think he took his time because he was a little bit more tired and that’s part of the game. It helps him a lot.”

Roddick wouldn’t go as far as Robredo, but he enjoyed rattling off Djokovic’s true ailments and a few fictional ones, including bird flu, anthrax, and SARS.

“You know, either he’s quick to call a trainer or he’s the most courageous guy of all time,” Roddick said. “I think it’s up to you guys to decide.”

Count me among those who believe that Djokovic is not well at the moment. Perhaps he’s a bit too demonstrative, but he certainly has abused himself in his last two matches (and his opponents have abused him). For all of Djokovic’s good qualities — excellent serve and all-around strokes, fast feet, a fantastic return of serve, and abundant determination — his balance and body control remain a weakness. One hardly ever sees Federer slip or fall, whereas Djokovic often seems as if he’s about to spill out of his shoes. When you lack control, injuries are more common. Roddick can exploit this weakness. If Roddick serves well, he need only make Djokovic work — rally after rally — to wear him down and force errors.

At this point, I wouldn’t be surprised, or, at least, I’d be only modestly surprised, if Roddick beat Federer, too. For every brilliant moment the four-time defending champion had in his five-set victory over Igor Andreev on Tuesday, there were two or three atrocious ones, at least by Federer’s lofty standards. On Tuesday, Federer looked more like the Federer whom Roddick defeated in Miami this year, rather than the one who has knocked Roddick out of this tournament the last two years. Andreev has had little success on hard courts in his career — even his best performance wouldn’t normally push Federer to five sets. Unless Federer lifts his game, Roddick has perhaps his best chance at reaching a major final for the first time in two years.

Andy Murray now has a chance to reach the first major final of his career, though he’ll need to play a lot better to do it. Murray yesterday outlasted Juan Martin Del Potro in a four-set, four-hour match, 7-6(2), 7-6(1), 4-6, 7-5.

Murray has more tricks up his sleeve than any player on the tour save Federer, yet rather than dazzle Del Potro with an array of precise forehands, bullet backhands, and angled volleys, he decided to play David to Del Potro’s Goliath (Del Potro is 6-foot-5). Del Potro hits the ball as hard as any tennis player ever has, and he slugged away at Murray hour after hour. Murray mostly chipped and sliced the ball back, tossed up a few lobs, dropped many drop shots, and waited patiently for Del Potro errors. The Murray who pushed opponents around on the hard courts this summer — the one with a 135 mph serve, blazing forehand, and attacking style — was nowhere to be found.

One can’t argue with the result: Twice Del Potro lead by a break of serve in the fourth set, but Murray broke him three times in return. By the end of the match, Del Potro could hardly walk as Murray tortured him with side-to-side strokes and drop shots. Del Potro was a broken man.

Unfortunately, the match took almost as much out of Murray. In the semifinals, he’ll need to play aggressively and shorten points, especially if his opponent is Rafael Nadal. Can Murray adjust his tactics that quickly, ditch his passive ways, and carry the day on Saturday? As Roddick and Muller have shown us, there’s hope. Fortunes can change that quickly.

Mr. Perrotta is a senior editor at Tennis magazine. He can be reached at tperrotta@tennismagazine.com.