1. bruguera def courier fo 93
2. costa def ferrero fo 02
3. johansson def safin ao 02
4. safin def sampras uso 00
5. hewitt def sampras uso 01
6. sampras def agassi uso 90
7. stich def becker w 91
8. kuerten def bruguera fo 97
9. ivanisevic def rafter w 01
10. courier def agassi fo 91
I don't really think matches where one player would eventually go on to prove that their "day in the sun" wasn't a fluke should be considered as biggest upsets. So, strike Bruguera-Courier, Hewitt-Sampras, Safin-Sampras, Stich-Becker, Bruguera-Kuerten, Ivanisevic-Rafter, and Courier-Agassi, etc. These are all top notch quality talents and guys I think history has shown. To me, someone like Stich or Kuerten laying the smackdown on Becker and Bruguera on their court isn't really that much of an upset, as those guys really are that good when they're on and in the zone. Likewise with Hewitt if he's facing a serve and volleyer like Sampras, who he was basically designed as the perfect counter to by God. A motivated Ivanisevic beating Rafter to me isn't really an upset. Courier vs. Agassi doesn't say much to me either, when Courier would go on to prove he really was the better clay court player, and so on and so forth.
To me, a big upset is not when you have two big names, but when you have but a mere PSEUDO name player like a Costa, Gaudio, Gomez, or Johansson in the finals (and unlike say a Stich-type, you KNOW that's all the talent they have in them...they don't truly have the talent to ever be considered among or vie with the elites of the game, i.e. say a top 5 ranking level standard) against a REAL name player, and they lay the smackdown.