Nadal is a vastly superior hard court player than Agassi.
Nadal had better movement, better forehand, better versatility, better drop shots, substantially better peak level.
Agassi had a better return rate and better backhand, but a subpar movement compared to Nadal.
Nadal won 3 out of his 6 Slams on hard (50%) facing either Federer (AO 2009) or Djokovic (USO 2010, USO 2013).
Agassi only won 2 out of his 6 Slams on hard (33%) facing Sampras. And those victories where against Sampras at the AO.
It is a well-known fact that both Federer and Djokovic are undoubtedly superior hard court players than Sampras. Both Federer at the AO and Djokovic at the USO are much better than Sampras at the AO. Ergo Nadal faced tougher competition.
Agassi won 4 out of his 6 Slam finals on hard (67%) against players that were NEVER #1: Schüttler, Todd Martin, Clément and Stich.
Nadal won only 1 out of his 6 Slam finals on hard (17%) against players that were never #1: Kevin Anderson.
Now, Federer and Djokovic fans have joined forces to hate on Nadal on this thread.
The agenda is clear: Federer fans wanna hype Agassi because otherwise it would be too embarrassing to admit that the mytical and hypothetically undestructible peak Federer struggled to beat ancient mommy Agassi with no mobility and a genetic back problem, the degenrative disc disease called spondylolisthesis.
On the other side of the agenda we have Novak fans. They love to diminish whatever they feel as a threat to Novak's greatness. As Nadal is the second greatest of all time, they try to diminish Nadal to further separate him from Novak.
They're both being 0% objective here due to the aforementioned reasons. They'll make excuses to justify Nadal leading the H2H over Federer at the AO and Novak at the USO. But all those excuses will be lost in time like tears in rain. Posterity will remember that Nadal is simply a better hard court player than Agassi, and was able to win as many Slams on hard as Agassi despite playing in the same era as the 2 best hard court players ever and thus facing tougher opposition than Agassi.
Here older Agassi was facing teenager underdeveloped "Hardal."
Agassi was closer to his prime than Nadal on hard courts here in Toronto 2005. Agassi just reached the US Open final a few weeks after this match, while Nadal didn't reach his first ever Slam final on hard till a 3 and a half years later in 2009. Still, Nadal convincingly beat Agassi. Say no more.