Probably the day he beat Sampras at Wimbledon, unless Pete had an off day. The last two Wimbldon finals certainly were great matches. Oops! I thought we were talking about great matches, not one sided ones. A great match though is one where a player has to play their best to beat another great player, playing their best tennis.
Judging matches you haven't seen eh? Interesting.
Anyway, I say either the Ancic quarter in 06 or against Nadal last year.