The American great, 40, returned to the same Centre Court where she had limped out injured last year with a passionate display in a 7-5 1-6 7-6 (10-7) defeat.
Willed on by a fervent crowd, Williams answered questions over her fitness with a dynamic display that put her two points from a win before Tan edged it.
The burning question now is surely 'was this her last Wimbledon'?
"That's a question I can't answer," she told a news conference. "I don't know.
"Who knows? Who knows where I'll pop up."