Clay has never been Roger Federer's strength. The statistics show that it's objectively his worst surface—although Rafael Nadal has something to do with that. It's also a surface that he has barely played for the last few years. But, the Swiss grew up playing on red clay, and he's back on it this year, starting in Madrid.
The Madrid draw may be stacked, but the world No. 3 will be a huge storyline this week regardless of the result. Though not his best, his clay accomplishments are still top notch.