Williams has won the US Open three times as part of her overall 14 career Grand Slams and Clijsters said that on form, the American is unbeatable.
"Serena is the best ever just because physically she just stands out," Clijsters said. "When she's in good shape she just stands out tremendously.
"I mean, she's fast, she's strong, she has a very good eye, as well. It's the combination. What we have seen over the last few months is the best player ever."
Clijsters is due to retire after this year's US Open, which starts on Monday.