Cuevas upset Rafael Nadal -- who used to be unbeatable on clay -- 6-7 (6), 7-6 (3), 6-4 on Saturday. It was the Uruguayan's first victory over Nadal in three attempts, and he finished off the match with an ace.
Nadal, the 14-time Grand Slam winner, was also beaten in the semifinals in Buenos Aires a week ago, also on his favorite clay surface.
"I'm really happy because Rafa is a great player," Cuevas said. "I came out here thinking I could win, and I knew I had to stay relaxed, which I was able to do for almost the entire match."
Pella won over rising Austrian star Dominic Thiem 6-1, 6-4.
Thiem won a tournament in Buenos Aires a week ago, beating Nadal in the semifinals, and had won eight straight matches before the loss.
Pella and Thiem played in heavy wind during the first set. Then the match was delayed for an hour in the second set by a downpour. None of it fazed the Argentine.
"The first set was terrible to play, but I was raised in a place where the wind was as bad or worse," said Pella, who grew up in the port city of Bahia Blanca in Buenos Aires province. "So I was used to playing in conditions like this."
Pella said he just tried to hit for center of the court, and aimed sparingly for the edges.
Thiem, the youngest player in the ATP's top 20, played singles and doubles on Friday. The doubles match finished about 1:30 a.m. -- and it showed.
"Of course it was strange in the beginning, but he had the same conditions and just handled it better than me," Thiem said.
Thiem overpowered Spain's David Ferrer in the quarterfinals 6-3, 6-2, but he was a shadow of himself against Pella.
"Everything I had was a couple percent less than I had the last days and the last matches," he said. "On this level when you don't have these last couple percent, you just go down immediately."
In the women's semifinals, Shelby Rogers of the United States defeated Sorana Cirstea of Romania, 6-4, 6-4, and Francesca Schiavone of Italy won over Petra Martic of Croatia, 6-3, 6-3. Rogers and Schiavone meet in Sunday's final.