Capitalising on the absence of Serena Williams, who is recovering from knee surgery, Clijsters seized the world number one ranking when she won the WTA title in Los Angeles earlier this month.
Although no-one denied the amiable 20-year-old such an honour, it was quickly pointed out that she was the first number one not to have a grand slam title on her resume.
As a character, Henin-Hardenne is harder to relate to, so much so that she has been compared to both Martina Hingis and world heavyweight boxing champion Lennox Lewis by media in recent weeks.
But while Henin-Hardenne has one major title to her name -- this year's French Open after a sweet victory over Clijsters in the final -- the world number three remains an enigmatic figure.
Victory at Flushing Meadows would help correct that imbalance, although the 21-year-old will have to overcome a great deal in New York, not least an unsupportive crowd.
They are expected to give Henin-Hardenne a rowdy reception to avenge her behaviour towards Serena Williams in the American's controversial semi-final defeat at Roland Garros in June.
Clijsters has won many admirers for her demeanour and, while once she may have been referred to just as Lleyton Hewitt's girlfriend, she is now a respected force on court.
SPECIAL FEELING
A grand slam title would turn her into a feared champion.
"It's a very special feeling to be number one," said Clijsters.
"It's nice to hear 'number one' when they announce you on court at the beginning of a match.
"But, of course, winning a grand slam motivates me very much. I like the challenge.
"The U.S. Open will be tough because without Serena playing, there will be a lot players who think they have a bigger chance to win."
Since Serena will not be defending her title, Clijsters' biggest threat in New York is expected to be Henin-Hardenne.
"She's the main challenger but there are other players who would like to get a win there too," said Clijsters.
"If I'm going to challenge her, it's going to be in the semis or final and I have to get there first.
"But if I play her, I'm definitely looking forward to revenge."
The Belgian duo have also recently engaged in a war of words after Clijsters accused Henin-Hardenne of using an injury time out as an excuse to regroup in the final of the Acura Classic earlier this month.
But Clijsters insists she and Henin-Hardenne are not enemies.
"Really, there is no problem with Justine, we just have two different characters," she has said.
However, the longer they both remain at the top of the game, and competing for the same honours, the more the rivalry will intensify.
