This is what happens when Justine Henin-Hardenne, the world's top-ranked tennis player, catches a cold.

"First, you talk to the hotel to make sure her room gets the perfect-temperature air conditioning," trainer Pat Etcheberry said. "Then you work on breathing exercises. You make sure she sits down most of the day and doesn't really move."

The top-seeded Henin-Hardenne survived her opening match at the U.S. Open on Tuesday, handling Nicole Vaidisova, 6-1, 6-4, but her problematic immune system remained a major concern. She had bronchitis in February, and she sat out most of this summer because of an energy-sapping virus. Now the defending U.S. Open champion has a cold, and she has taken it very seriously.

All of the big names advanced Tuesday, except 2000 U.S. Open champion Marat Safin, who fell to Thomas Enqvist in a four-set match. Henin-Hardenne's win, though, registered as the most impressive. Because of the cold, she practiced for just 30 minutes combined during the two days before her opening match.

"I'm starting to feel a little bit better," she said. "After the way my health's been the last few months, I have to be very careful. That's my only worry."

She looked sluggish sometimes, especially during the second set. Her first serve, normally one of the game's most consistent, rarely went in. Vaidisova, a 15-year-old making her Open debut, broke Henin-Hardenne twice in the second set and took a 4-1 lead. "For a minute there," Vaidisova said, "I thought I might have her. She was making more than her usual amount of mistakes."

But Henin-Hardenne recovered in distinctive fashion, using flawlessly placed groundstrokes to come back and win the set. On match point, she painted the baseline with a cross-court backhand that Vaidisova had no chance to return.

"I'm happy with how I recovered," Henin-Hardenne said. "She definitely pushed me for a while there, which made things tough. I'm not quite back to 100 percent."

Then again, she hasn't felt fully healthy for six months now. Etcheberry, Henin-Hardenne's trainer, has grown used to monitoring illnesses. "She's had a stretch of several months now," Etcheberry said, "that's just been extremely unusual."

After winning the Australian Open in late January, Henin-Hardenne caught bronchitis. She came back quickly -- too quickly, Etcheberry now thinks -- only to catch a mononucleosis-like virus midway through April. Over the next three months, she played just once, losing in the second round of the French Open.

Her poor play in France persuaded her to return home to Belgium and focus on recovering. "I'd run for one day," she said, "and then stay in bed for the next three." She started practicing regularly again at the beginning of August. Two weeks later, she flew to Athens for the Olympics, still weak, rusty and tired.

She swept all six of her matches and won gold.

"I didn't think that would be possible from anyone," said Jennifer Capriati, the No. 8 seed. "The way she played over there was kind of shocking."

The result, though, was troubling and predictable: Henin-Hardenne left Athens feeling sick, just as she had for most of 2004.

Her success here will likely depend on how long her cold symptoms linger. The next two weeks are supposed to be hot and humid, conditions that will exaggerate any fatigue Henin-Hardenne already feels. If she recovers quickly, she will be the favorite to win this tournament. She won the French Open and the U.S. Open in 2003, while reaching the semifinals in both other majors.

Her spin-heavy serve can be menacing. Her backhand, a powerful shot she hits mainly with her wrists, may be the best shot on the women's tour.

"She's not far from that kind of level now," Etcheberry said. "Absolutely we'll be keeping a close eye on her. After what she's been through, you can't just let a cold go and not pay attention. It would be nice if she could finally feel totally good again."

"After the way my health's been the last few months, I have to be very careful," says Justine Henin-Hardenne.