Rafael Nadal struggled into the Brazil Open final on Saturday by downing Argentine lucky loser Martin Alund in three sets and later said he was handicapped by nagging knee pain.
The former world number one, who is recovering from a left knee injury, battled hard to dismiss his 27-year-old opponent, who is ranked 111 in the world, winning 6-3, 6-7 (2/7), 6-1 in nearly two hours.
Sunday's final will pit the 26-year-old Spanish star against Argentine David Nalbandian, who whipped Italian Simone Bolelli 6-3, 7-5 in 85 minutes in the other semi-final.
It will be Nadal's second singles final since his comeback from a seven-month absence due his left knee injury and then illness.
Last week, he competed in the Vina del Mar Open in Chile, losing the singles and doubles finals.
At a post-match press conference, Nadal said that while he was physically prepared for Sunday's final, "my knee is not".
"I don't think I am the favorite at all. The conditions are much more favorable for David (Nalbandian)," he said. "I still have discomfort. 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."
The 31-year-old Nalbandian, a former world number three who now languishes in 93rd place in the ATP rankings, has also been plagued with injuries.
He withdrew from the 2012 US Open due to a strained muscle in his chest.
In Saturday's semi-final, Alund more than held his own against Nadal, prevailing in the second set before the world number five stepped on the gas and sealed his victory with a masterful performance in the decisive third set.
The duel between Nadal and Nalbandian will come immediately after the doubles final, which will pit the pair of Czech Frantisek Cermak and Slovak Michal Martinak against Austrian Alexander Peya and Brazilian Bruno Soares.
Nadal, who has 11 Grand Slam titles under his belt, won the Brazil Open in 2005, when it was held in Costa do Sauipe in eastern Bahia state.
Last year, the tournament was moved to Sao Paulo.
The Brazil Open is part of the Latin American clay court swing, along with the Vina del Mar event and the Mexico Open, in Acapulco, where Nadal plans to compete later this month.
The three low-profile Latin American events are routinely ignored by the world's top three players – Novak Djokovic, Roger Federer and Andy Murray.