He also played a guy named Djokovic at the Australian Open. I'm not sure his level dropped so much as Djokovic and Nadal improved, and he hit one of his low points against Nadal.
This view is not supported by the actual results. Roger started losing to lesser players more frequently in 2007. Look at who he lost to, by year:
2004: Henman, Nadal, Costa, Kuerten, Hrbaty, Berdych
2005: Safin, Gasquet, Nadal, Nalbandian
2006: Nadal x4, Murray
2007: Canas x2, Nadal x2, Volandri, Djokovic, Nalbandian x2, Gonzalez
2008: Djokovic, Murray x3, Fish, Roddick, Nadal x4, Stepanek, Simon x2, Karlovic, Blake
In 2004, he wasn't a great clay-courter yet, thus the losses to Costa and Kuerten. The real travesty was losing to a 19-year old Berdych at the Olympics.
In 2005, Safin played the match of his life to win the AO, and while he had improved on clay he wasn't great yet - thus the Gasquet loss. Rafa doesn't need explanation, and you know better than anyone that Nalbandian can play brilliantly at times. But still, 4 losses.
In 2006, he was almost impossible to beat, except his arch-nemesis Rafa and this upstart Andy Murray. It remains his best overall year, and his true pinnacle.
In 2007 he lost to a few players he had no business losing to: Canas twice, Volandri, Gonzalez. Not much of a drop from 2006, but still there. My guess is his actual level didn't drop as much as, perhaps, his focus.
And of course no one will deny that his level dropped substantially in 2008, with 15 losses: as many as 2004-06 combined! While he did lose 8 of those to Rafa, Andy, and Novak, he also lost twice to Simon, and once each to Fish, Roddick, Ivo, and Blake - peers of his he normally dominated.
Anyhow, as I said, I think he lost a hair in 2007, although probably more due to slight loosening of focus. The bigger drop came in 2008...whether that was because of mono or not, he never fully recovered and returned to his 2004-07 form.
