My measurement of Great is: 1/ Even match up of 2 great players playing their best over a sustained period of the match. 2/ Where you don't know what the final result will be until near the end of the match 3/ Amazing shots played at tense moments 4/ A Match dripping with Intensity My Opinion Clay: Italian Open final 2006 - Nadal vs Federer Grass: Wimbledon final 2008 - Nadal vs Federer Hard: ????? I find this hard to determine. Some people would say Agassi Sampras 2001 US Open but I'm at not all sure - be interested in what people think. Indoor : I didn't see it but perhaps Federer Nalbandian - Masters Cup 2005 ? What do people think were the greatest matches by surface of the 2000's?