All players have their off days, but most of the top girls manage to find a way through it. But some seem able to lose to lose to so low-ranked players they should really beat in their sleep. I think the main ones are:
Daja is not "anyone". Monica played her worst healthy match in about 3 years, but I guarantee you she would have beaten at least 7 of the 10 players ranked between 31 and 40 anyhow. She would have found a way against someone less talented, or less determined, or less familiar with her.
I think Venus and Serena Fan is referring to Amelia Island '2000
Serena withdrew against Paola Suarez and was losing badly before she withdrew, since that time the only players Serena has lost to ranked lower than her are Monica and Megahn (another withdrawl) its a pretty incredible record.