Osaka has admitted feeling burdened by expectations after last month's stunning victory in New York, saying she puts pressure on herself to "do more, do better".
Williams, 37, brought a halt to her season after the US Open final, where her latest attempt at a record-equalling 24th major title ended in acrimony.
"Looking at the players from the sidelines, we might not really understand some of their psychological goings-on on the court," said Li.
"I've always thought she is a very strong player. There are very few players who can give birth and then immediately come back out," she added.
"For me, it's really hard to find a balance between family and the tennis court, but she very quickly got into the US Open final." Li is often credited with igniting tennis interest in Asia and especially in China, but she said the sport still had a long way to go.
"Tennis in Asia is still in its adolescence. We have just passed the infancy stage, so we can still go a lot further," said Li.
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