As we all know, Sampras led his rivalry with Agassi 6-3 in Grand Slams. Agassi was 3-0 against Sampras at the Australian Open and French Open and Sampras was 6-0 against Agassi at Wimbledon and the U.S. Open. Two thirds of their matches were on surfaces that favored Sampras. Perhaps, the rivalry would have looked different if two thirds of their matches were at the Australian Open and French Open.
Similarly, Nadal is an incredible 8-2 against Federer in Grand Slams. Federer is 2-1 against Nadal at Wimbledon. Nadal is 7-0 against Federer at the Australian Open and French Open. 70% of their matches were on surfaces that favored Nadal. Perhaps, the rivalry would have looked different if 70% of their matches were at Wimbledon and the U.S. Open.
In the Sampras-Agassi case, their GS hard court matches could have gone either way (bar the very first one), in my opinion it did not depend so much on it being rebound ace or decoturf II, but on other factors:
Their first GS hard court meeting: 1990 USO final: Sampras won 6-4 6-3 6-2. Okay, this is the only one not close.
1995 AusOpen final: Agassi won 4-6 6-1 7-6 6-4. Sampras played three consecutive extremely long and hard fought matches in the rounds R16 (defeated Larsson by 4-6 6-7 7-6 7-5 6-4 ), QF ( defeated Courier by 6-7 6-7 6-3 6-4 6-3 ) and SF (defeated Chang 6-7 6-3 6-4 6-4 ), add the summer extreme heat of Melbourne, and Sampras was quite spent in the final. In spite of that, he was 6-4 up in the third set tie-breaker to get a two-sets-to-one lead (though he would have lost in five either way, he did not have fuel enough in the final given the circumstances).
1995 USO final: Sampras won 6-4 6-3 4-6 7-5. Could have gone either way too. Agassi played the second SF the night before (a tough and very close match against Becker) and the match could have gone different in many aspects. Agassi was holding serve easily during the first set till the 10th game, when Sampras suddenly played several great points from the baseline (and Agassi made some stupid mistakes) and broke to get the first set. Sampras got an early break in the second set, but when he was serving 5-3 Agassi had two break points opportunities to level the set. He was a bit unlucky again. Then Agassi won the third set 6-4 and the fourth set was 7-5 Sampras. A very close match that could have gone different easily and Agassi was maybe a bit more tired than Pete because of playing the second SF the night before.
2000 AusOpen SF: Agassi won 6-4 3-6 6-7 7-6 6-1. Again, a very close match that could have gone either way (in fact Sampras was two points away from victory more than once during the fourth set). Agassi survived the fourth set and Sampras crumbled in the fifth set (lack of stamina as usual in him and some injury that he got through the match if we are to believe him). A very close match that could have gone either way.
2001 USO QF: Sampras won 6-7 7-6 7-6 7-6. No need to comment. Could have gone either way. Very few points here and there decided the match in the four tie-breakers.
2002 USO final: Sampras won 6-3 6-4 5-7 6-4. Sampras saved a lot of break points in two different games through the fourth set that could have meant losing the fourth set and then I am sure he would have lost the match in five (Agassi was fitter than Sampras, especially at that stage of their careers), but almost miracously he saved those games, and then broke Agassi nearly the end of the fourth set and served to win it all, and did it. Besides, Agassi again played the second SF the day before and started the final a bit flat, losing the first two sets in a combination of Sampras playing great and Andre being a bit flat. A match very close that again could have gone either way.
So in my honest opinion, it did not matter rebound ace or decoturf II, all their hard court GS matches (bar the first one) were very close and could have gone either way. They were decided by other factors (heat, tiredness of one or the other because of long previous matches or second SF in the USO,...), more than the surface in itself.
