Here are the stats for chang vs agassi as far as returning is concerned . I checked from the atp website regarding the top 10 and top 20 players faced ..
The last row shows the percentages
I removed the david cup matches for both because I didn't the rankings for those from the atp website
as you can see from this, agassi played quite a bit more against the top 10 ( more than 3% difference ) and against the top 20 ( close to 6% difference )
Makes a huge difference ...