US Open Series leader is a title Sam Querrey better not have gotten too used to, because it's unlikely he'll be holding on to it much longer.

Querrey, who won Los Angeles and made the finals of Indianapolis before bowing out respectably to Andy Roddick in Washington, suffered a surprising loss to Germany's Philipp Petzschner in the first round of the ATP 1000 Masters event in Montreal, the Rogers Cup.

The match was over in a brisk 65 minutes. Querrey never had a break point on Petzschner's serve, and lost all three break points he faced on his own service.

I only saw a few minutes of this match, but it was clear that Sam was not happy with how he was playing (shocking, I know). His body language was awful, and he seemed frustrated to have no answers. There didn't seem to be any one part of his game that was letting him down (maybe his return), it just looked like mental fatigue.

Querrey had been tagged by many as a US Open dark horse to watch, but now the discussion should change to whether or not he peaked too early.