Champion Justine Henin-Hardenne admitted her effort in winning the gold medal at last month's Athens Olympics could make it tough for her to retain the U.S. Open title.

The world number one limped into the third round of the U.S. Open on Thursday after she eked out a 6-2, 5-7, 6-2 victory over Israeli Tzipora Obziler, the world number 133.

"I can see that most of the players who played well at the Olympics have had many problems at the beginning of the tournament," said the Belgian top seed.

Playing her first tournament in 12 weeks after being sidelined by a mystery viral illness, Henin-Hardenne surprised many by triumphing in the Greek capital.

"I can tell you that it's not easy coming from Athens. Really tough conditions, a big tournament, the Olympics. It's a different kind of experience," she said.

"And then a week later you are again in a Grand Slam. There's a lot of pressure.

"It was a big surprise for me that I won the Olympics. But it took a lot of energy the last weekend in Athens. I didn't sleep enough. It's going to be a big test for me but you can't be at your best all the time."

The top-seeded Belgian was completely out of sorts against Obziler, losing her serve seven times and making 47 unforced errors but survived to set up a match with American Lisa Raymond or Russian Maria Kirilenko.

The 31-year-old Obziler, playing only her second Grand Slam event after almost quitting the sport following two years off with a shoulder injury, caused Henin-Hardenne all sorts of problems.

The Belgian was broken when she served for the match in the second set and she had to dig deep before in the third set before advancing to round three.

"She didn't give me any rhythm today," Henin-Hardenne said. "Most of the players on tour hit the ball hard all the time but she used her slice well and it was hard for me to find my rhythm."