Japanese top seed Kei Nishikori lost a sixth successive final on Sunday when he was defeated by Ukraine's Alexandr Dolgopolov 7-6 (7/4), 6-4 in the Buenos Aires clay-court title match.
The world number five had already lost in the Brisbane final in January to Grigor Dimitrov which followed 2016 defeats in tournament deciders in Basel (against Marin Cilic), Toronto (Novak Djokovic), Barcelona (Rafael Nadal) and Miami (Djokovic).
"I'm happy to be in the final. Unfortunately I lost today, but Alex played better than me. I didn't play badly, so hopefully I can keep this level going," said Nishikori.
For world number 66 Dolgopolov, it was a third career singles title and first since Washington in 2012.
"I think you can call it a perfect week. I didn't lose a set here and beat Kei for the first time," said the 28-year-old.
Nishikori was seeking a 12th career crown but he will get another opportunity next week when he plays as top seed at the Rio clay-court tournament.