John McEnroe did not say that any high school boys tennis player at a halfway decent program could beat Serena William. STFU.
If match then 1/8 is probably about right. She has to make a mistake at some point 1/8 matches. It game has to be much much lower than 1/8.
The track analogies are not good given that you don’t make “unforced errors” in track. That would be more in line with 1/8 men think they could beat Serena Williams which is not what this is.