By Emma Davis

CHARLEROI, Belgium (Reuters) - World number two Justine Henin-Hardenne said on Sunday the knee injury that forced her to retire from the decisive match of the Fed Cup final against Italy was serious.

The French Open champion suffered the injury in her win over Francesca Schiavone in Sunday's opening match, but despite her obvious pain, started alongside team mate Kirsten Flipkens in the doubles decider.

Henin-Hardenne and Flipkens took the first set 6-3 but lost the second set 6-2 and trailed 2-0 in the third when Henin-Hardenne retired.

"I had given it my all in the singles and I didn't have enough left for the doubles. It was difficult. I have to turn the page now and take care of myself because this injury is serious," she said.

The Belgian had been wincing with pain throughout the match but landed heavily early in the final set, leaving Italy to snatch their first Fed Cup title.

"This is an injury that I've had for about a year and a half and it's something that's going to come back. It's a problem with the cartilage and I have to take care of it," she said.

Henin-Hardenne has had a year of ups and downs after making it to the finals of four grand slams, scooping victory at the French Open but retiring from the Australian Open final due to stomach pain and pulling out of the Fed Cup semi-final.

She had recovered from the bad press surrounding her withdrawal from the semi-final, due to exhaustion, to find a stadium filled with roaring fans in Charleroi, but victory again proved a step too far for the injured 24-year-old.