Monfils is just not a fighter. In fact, I'm pretty sure he can only win a match if he's cruising despite all his gaffes and fooling around. Has he ever won a tight match were he didn't seem way better than the other guy anyway?
Probably not. Not to mention he's lost matches where he seemed like the better guy ( like last week against Mathieu ).
But he was careless today and it didn't seem like he cared much.