It makes me wonder though, what constitutes 'a bad matchup'? For example, people say Nadal's a bad matchup against Federer, but not nearly as many people say that Federer's a bad match up for Murray or Nadal for Dimitrov or Djokovic for Del Potro. Just what/when is something a bad match up?
A bad match-up is the situation were the interaction of playing styles of two players doesn't allow the superior player to win as would be expected. The court conditions have a huge effect on match-ups.
For me, players have roughly the same level than some players, are inferior to some players, and are superior to some players. E.g., Federer, Djokovic and Nadal are roughly equal, they are superior to everyone else. Tsonga is inferior to them, roughly equal to other top 10-20 players, superior to everyone else.
Now the result of a match between two players depends primarily about the comparison of their level AND of how their game styles match-up with each others AND how their game styles interact with the court conditions.
So generally when a superior player beat an inferior player, you don't go to far into match-up because the result was expected (Nadal vs Dimitrov).
If two roughly equal players have a balanced H2H, you don't go to far into the match-up because it is expected than equal players will split results. If the H2H is unbalanced (Nadal-Federer), then you need to look at match-up to understand why a player can consistently dominate someone who is roughly his equal.
If an inferior player beat a superior player several times, this is an unexpected result and you need to look at the match-up: how does the game style of the inferior player allows him to defeat a superior player?
For me there is no doubt that Wawrinka is an inferior player to Djokovic, and his success at slams against him is a surprise, including his close 5 sets defeats in Australia or New York, because if I consider only their playing level, Wawrinka should not regularly push Djokovic to five sets, nor win!
Great exemples of match-up are thus Wawrinka-Djokovic, Davydenko-Nadal, Nadal-Federer (now reversed). You could also argue that Federer-Roddick is an exemple of bad match-up, because the H2H gap is larger than the level gap between them.
