The former world number one set up a singles final showdown with former doubles partner David Nalbandian - but admitted his troublesome knee was struggling during a two-our semi-final match with Martin Alund.
Nadal reaches Brazil Open final despite return of knee problems
Rafael Nadal struggled into the Brazil Open final and a showdown with former doubles partner David Nalbandian - but was plagued by more problems with his knee.
The former world number one, who missed seven months of last season due to injury to his left knee, had already withdrawn from the doubles competition in Sao Paulo.
Nadal needed nearly two hours to beat unheralded Argentinian Martin Alund 6-3 6-7 (2/7) 6-1, just 24 hours after beating Carlos Berlocq in three sets in the quarter-finals.
And afterwards he admitted that while he felt physically ready for the final, his knee did not.
"It was the worst day for my knee, and if the knee is not well it makes it very difficult to play," he said.
"I don't think I am the favourite at all. The conditions are much more favourable for David. Today the knee did not respond well. And in those circumstances, not being able to move well, to attack the ball, it will be very difficult."
"I am going to do what I can ... But I think it is going to be a very complicated match for me."
Nalbandian beat Simone Bolelli 6-3, 7-5 in 85 minutes in the other semi-final.
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