They were the talk of tennis a mere seven months ago: two flamboyant, up-and-coming stars of the game. One, Novak Djokovic, had dreamed of being no. 1 in the world since tennis courts were built near his home in the mountains of Serbia. The other, Jo-Wilfried Tsonga, a 200-pound specimen and master of the drop volley, had overcome several injuries and begun to fulfill the promise he showed as junior. When they met in the Australian Open final, they lit up the Melbourne evening with splendid shots, frequent fist pumps, and telegenic smiles.

Melbourne seems like so very long ago, doesn’t it? These days, Djokovic and Tsonga are just trying to stay in the picture as Rafael Nadal receives all the attention. In Melbourne, Tsonga demolished Nadal in the semifinals with a brand of power tennis not seen, perhaps, since a young Marat Safin mauled Pete Sampras in the 2000 U.S. Open final. Not long after, Tsonga hurt his knee, which required surgery. He hasn’t played since May. Djokovic is healthy, but he seems to have lost a bit of his swagger, especially against Nadal, who has beaten Djokovic four times this year, twice on hard courts, once on clay, and once on grass. Near the end of the clay court season, Djokovic was one match from knocking Nadal out of the no. 2 spot in the rankings. Instead, Nadal held on, won the French Open, won Wimbledon, and then zipped past Roger Federer to take over the no. 1 ranking, something few people would have thought possible a year ago.

At the U.S. Open yesterday, Djokovic and Tsonga kicked off their campaigns with convincing first-round victories. Djokovic, the no. 3 seed, beat Arnaud Clement, a Frenchman with exceptional hands and lots of power for a man with such a slight build, 6-3, 6-3, 6-4. The only scare for Djokovic came in the third set, when he turned his right ankle and received treatment from a trainer. The Serb played hesitantly for a few moments, but he later said his concern was greater than the pain warranted.

Tsonga started more slowly against Santiago Ventura, a 28-year-old from Spain who, oddly enough, was the last man Tsonga played before his surgery. In Australia, Tsonga seemed to float around the court as he thumped one forehand winner after another. Yesterday, he said, he favored his knee a bit and was far less explosive in the early going. His form never came close to what it was seven months ago, but it was good enough for a 6-7(3), 6-4, 6-2, 6-3 victory.

If healthy, Tsonga and his mighty strokes would have an outside chance at this tournament. Unfortunately, Tsonga doesn’t seem so certain about his knee, even after so much time away.

“I’m a lit bit worried,” he said. “And of court it’s different than before. But I feel good and I can play tennis, so I play and I see.”

Djokovic, who lost last year’s U.S. Open final to Federer, is a favorite. His well-rounded game and athleticism are perfectly suited to these courts. One concern, though — and this is something I’ve noticed since Australia — is Djokovic’s tendency to succumb to frustration during a difficult match. It happened against Nadal in Hamburg. At the French Open, when he met Nadal in the semifinals, he never seemed to believe he could win. At Wimbledon, he played tentatively against Marat Safin and let his mood sour too soon. This summer, Djokovic twice lost to Andy Murray, whom Djokovic had beaten in their four previous meetings.

These instances of impatience and negative emotion are all the more noticeable because Djokovic is, usually, a spirited fighter and a tireless believer in his own talent. Tenacity is one of his strengths. In Australia, patience was one of his virtues, too, especially against Tsonga in the final. Djokovic had a lot on his shoulders in that match — it’s not often that a player reaches his second major final and is the hands-down favorite. He dealt with the expectations, and an onset of leg cramps, as only a champion could. Yesterday, he said he was not impatient, not in the least, to hoist another major trophy.

“No, no, no,” he said. “I just want to go slowly, step by step.”

The lesson of Djokovic and Tsonga is how quickly fortunes change in tennis. Before this tournament began, Nadal reminded reporters that a mere six months ago, most of us were asking him why he wasn’t winning any titles. (He didn’t win one between last year’s final in Stuttgart, Germany, a clay court tournament after Wimbledon, and this year’s Monte Carlo Masters, also on clay.) Remember Kei Nishikori? The Japanese teenager attracted a lot of attention when he defeated James Blake in Delray Beach in February. Since then, he’s lost nine matches and won five, including his first-round victory here. Marcos Baghdatis, who reached the Australian Open final two years ago and took part in a memorable U.S. Open duel with Andre Agassi in 2006, has an injured wrist and cannot play here. His ranking has fallen to no. 46 from a career high of no. 8 in 2006.

Of course, fortunes can change for the better, too. Both of these men learned that in Melbourne, and wouldn’t mind learning it all over again.

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