The 32-year-old Chinese relinquished a 5-2 lead in the first set to eventually lose the contest 7-5 6-1.
"I don't think I was playing badly, she just started playing a little bit better after she was 5-2 down," Li told reporters.
"I don't think I had the wrong game plan today or played wrong - I think it was still a pretty good match.
"And I was trying so many things today, like coming to the net more, so it's not bad, really."
Li is currently number two in the WTA rankings, while Williams is number one.