Rafael Nadal, via d.yimg.com

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After facing fairly unproven opponents in their first matches, the top four players in the world all face significantly tougher tests in their second matches in Montreal, in what is for each of them the first tournament since Wimbledon.

In his second match since Wimbledon and becoming a father, #1 Roger Federer faces countryman Stanislas Wawrinka, with whom he won the gold medal in doubles for Switzerland at the Beijing Olympics a year ago this week.  Wawrinka defeated Federer in their last match, in Monte Carlo, and has more matches under his belt since Wimbledon than the top Swiss, who in Wawrinka is facing his first opponent inside the top 400 since Wimbledon.

The player who goes into Thursday the least proven is #2 Rafael Nadal, who only had to play seven games against David Ferrer before Ferrer dropped out of the match due to knee tendinitis, the same ailment that kept Nadal out of Wimbledon.  Nadal faces Philipp Petzschner, who, while not a household name, already registered nice victories in Montreal over US Open Series leader Sam Querrey and #15 seed Tommy Robredo.  Petzschner hits the ball as hard as anybody, and won't give Nadal a ton of balls to look at if he's firing well.

After easing through Jeremy Chardy, #3 Andy Murray faces Juan Carlos Ferrero, who has already survived Lleyton Hewitt and Gael Monfils to make the third round, matchups that should serve the Spaniard well in preparing for Murray.  Murray crushed Ferrero in their quarterfinal at Wimbledon, but Ferrero has looked sharp in his hard court wins so far in Washington and Montreal, and has significantly more matches under his belt in the last month than Murray, whose main competition between Wimbledon and Montreal came at County Week in Eastbourne.

#4 Novak Djokovic looked unimpressive at best in his 6-4, 7-6(6) victory over wild card Peter Polansky in the first round, having to save a set point against the unheralded Canadian before winning in straights.  Djokovic faces former US Open semifinalist Mikhail Youzhny in his Thursday match, perhaps drawing the easiest opponent of any of the top four.  But Djokovic has looked anything but his former self in 2009, and is in danger of being ousted from the top four by Andy Roddick at the US Open if his results don't improve.

Most of the matches outside the top four look pretty juicy as well.  Roddick-Verdasco, Tsonga-Simon, and Davydenko-Gonzalez are all too close to call, in my mind.  The only clunker should be del Potro-Hanescu, which the Argentine should win easily if his mind is in it from the get-go.

So with rust and possible injury in mind, the question is: which of the four do you think is most likely to lose Thursday?