She is far and away the best player, but styles make match-ups. Everyone is beatable in tennis on a given day. Serena has had a bunch of 3 set matches recently..damn near lost to Watson.
I dont see her losing either, but she will eventually. Watson had the blueprint IMO. Very mobile, played really smart and made serena move vertically a lot. The best way to beat her is to drop shot her a lot and sucker her into the net then be precise on lobs and passing shots. Its a gamble, but no one in the world can out-rally her from the baseline. Even if a player does that and loses a majority of the points, it could tire serena out and open the door because serena (and all players) has a tendency to get flat-footed when tired. So if you slice a lot and keep the ball out of Serena's strike zone..hit it flat to her backhand out wide and drop-shot a bunch..that takes some of the power potential out of her swings and normalizes things.
Typical weak top spin rallies from the baseline is an automatic L vs Serena because she can catch it on the rise or get it in the wheelhouse and punish it without remorse. Thats how she breaks players serves so often because they hit 80mph kick serves, which usually end up slower off the bounce (less than 60mph) and is slower than a ground stroke.