Jonathan Rea is now 29 points behind rival Toprak Razgatlioglu in the World Superbike title race after the Turkish rider won the first feature race of the weekend in Argentina.
Razgatlioglu led from start to finish and clinched victory by over five seconds from defending champion Rea, who is still hoping to win his seventh successive title.
It was Razgatlioglu's 11th race win of the season, the same number as Rea, who will now be hoping to gain ground in the Superpole and second Superbike race on Sunday ahead of next month's final round in Indonesia.
"I made a really good getaway and tried to do my rhythm and focus on myself," said Rea.
"Toprak had a really strong pace and in some ways this was a research race for us.
"Hopefully on Sunday we can make some changes to the bike to be challenging nearer at the front."
Michael Ruben Rinaldi in third place with Alex Lowes was fourth and Northern Ireland's Eugene Laverty 13th.