Can be any number of matches or 1 per year (or slam), if you like
Sophie hearts Rafito.Yeah, the Samps-Agassi U.S.O. 2001 is the one that immediately springs to mind. Fed-Hewitt at Wimbledon 2004 was pretty good too, as was Tsonga-Fed this year.
And of course there was Gonzalez-Nadal at A.O. 2007, Murray-Nadal at A.O. 2010, & Ferrer-Nadal at A.O. 2011, not to mention Youzhny-Nadal at U.S.O. 2006.
2003: Roddick vs El Aynaoui AO
2003: Philippoussis vs Federer Wimbledon
2004: Gudia vs Coria FO
2005: Federer vs Safin AO
2008: Nadal vs Federer Wimbledon
2009: Nadal vs Verdasco AO
2009: Federer vs Soderling FO