he's always been a mediocre player that wins on athleticism alone
he does not have any distinctive shot...the only way he manages to win game is by running and keeping the ball in play

so when he runs out of gas like he did vs Robredo, he loses. It's that simple. Nothing to do with heart or not wanting it or anything like that, that's just cliches people like to spout out. He's just not that good, period.