The wins most point to when trying to hype Sampras' clay court resume are Muster in 1991, Bruguera and Courier in 1996, his Rome title in 1994 over Becker, and his DC win over Kafelnikov in 1995. Admittedly good wins.(But not the great wins most make them out to be) But in sports, it is often not only who you can beat,but who you can lose to that is just as important.
Let's look at who he managed to lose to during his world #1 years on clay (1993-1998 )
1993-lost to Jacco Eltingh (World #87)
1994-Didn't beat a top 10 player for his Rome title. Beat a good, but not great Boris Becker(World #13) in the finals. Outside of that, no one top 30 for the title. Beat opponents ranked #109, #283, #226, and #25 Paul Haarhuis to make the QF, where he lost to Jim Courier (World #7). This was probably his best CC season, and even still, nothing to write home about.
1995-lost to Olivier Gross (World #84), Fabrice Santoro (World #39) in warm up tourneys, before losing to Gilbert Schaller (World #24) in FO 1st Round.
1996-lost to Bohdan Ulhrich (World #38 ). Beat Bruguera (World #23) and Courier (World #8 ) to make FO SF, where he was smoked by Kafelnikov.
1997-lost to Magnus Larsson (World #43), Jim Courier (World #23), and then Magnus Norman (World #67) in the FO 3rd round.
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eat Agassi (World #21) in Monte Carlo, but subsequently got double breadsticked by Santoro (World #25) the next match. Won Atlanta on clay, but beat no one ranked above #40 to win that title. Lost to Ramon Delgado (World #97) in FO 2nd Round.
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So, I think that judging from Pete Sampras' career on clay, it is obvious to any objective person that he just did not have what it takes, as some others have mentioned, to win 7 consecutive matches against high level opposition on clay, no matter the era. The bad losses he has on clay far outweigh the good wins he has on clay.