Retiring star Kim Clijsters has hailed Serena Williams the greatest women’s player in tennis history.

Clijsters will put down the racquet after the US Open, which starts on Monday, content with her three titles at Flushing Meadows plus the 2011 Australian Open crown and two brief stints as world No.1.

The 29-year-old is the sentimental favourite entering her professional swansong and may yet add a fourth title to her US Open credits, but says there is no doubting who is the genuine favourite to reign in New York.

“To me, Serena is the best ever just because I think physically she just stands out,” Clijsters said on Saturday.

“When she’s in good shape, I think she just stands out tremendously. I mean, she’s fast, she’s strong, she has a very good eye as well.

“I think the combination of that is what we have seen over the last few months – (she) is the best player ever.”

Since April, Williams has won 34 of 36 matches, seized her 14th grand slam title at Wimbledon and an Olympic singles gold medal with a 6-0 6-1 mauling of French Open champion Maria Sharapova.

When Clijsters burst onto the scene as a 15-year-old back in the late 1990s, the Belgian considered Steffi Graf and Monica Seles her idols.

But now she believes Serena is in a league of her own.

“I was in full admiration of all those big names,” Clijsters said.

“I think now it’s a different story. I do my thing and I’m here to play tennis and that’s it. Then I go home to the hotel; whereas when I was younger, you like to see what it’s like to be in the same locker room as those big names.

“I feel very lucky that I have been able to be a part of a big generation where Venus and Serena kind of raised women’s tennis to a completely different level.

“So it’s been an absolute honour to be a part of that.”