It looks like there's only one official match between Pete Sampras and Roger Federer (their meeting in the 2001 Wimbledon). But they have also played at least 4 exhibition matches against each other: Federer went 2-1 against Sampras in an Asian series in 2007, and Federer beat Sampras at Madison Square Garden in 2008. Did they play any other exhibition matches against each other?
Those are the only 4 exhibition matches they've played against each other and you're correct - they only played 1 time in their careers at Wimbledon in 2001 where Federer won in the 5th set 7-5.
They did play doubles together (so, not against each other) in an exhibition match at the Indian Wells tournament in 2010 against the team of Andre Agassi and Rafael Nadal to raise money for Haitian earthquake victims.