Tennis stars Lleyton Hewitt and Kim Clijsters are set to marry after he proposed during a romantic cruise on Sydney harbour on Tuesday.

The Australian former world No.1 and the Belgian world No.2, who also held down the top ranking for more than two months this year, are officially engaged, according to The Daily Telegraph newspaper.

Sydney radio station 2UE also reported the engagement had been confirmed by Hewitt's management company Octagon, and said no wedding date had been set.

"It was a special evening together in one of the most beautiful cities in the world," 22-year-old Hewitt told the newspaper.

"We couldn't have asked for more."

Clijsters, 20, was said to be overjoyed.

"Lleyton did surprise me a bit but I couldn't be happier," she said.

"He has always been one to pay great attention to detail but I think he outdid himself this time."

Hewitt's father, Glynn, said both sets of parents were "very pleased".

The pair have dated for almost four years and are the most high profile couple on the world tennis circuit.

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