Latvia's Martins Plavins and partner Edgars Tocs beat Brazil's Alison Cerutti and Alvaro Morais Filho in the men's beach volleyball quarter-finals on Wednesday.
The Latvian pair had defeated Brazil's Bruno Schmidt and Evandro Oliveira on Monday.
Since beach volleyball became an Olympic sport in 1996, Brazil and the United States have dominated the event but countries like Latvia are now challenging their supremacy.
In another surprise, Qatar's Cherif Younousse and Ahmed Tijan beat Italy's Paolo Nicolai and Daniele Lupo, the 2016 silver medallists, 21-17 23-21.
They will face Russian Olympic Committee (ROC) duo Viacheslav Krasilnikov and Oleg Stoyanovskiy in the last four. Women's pair April Ross and Alix Klineman are the only American team left in the tournament.
"Because they (European countries) are opening indoor facilities for beach volleyball and they are getting more and more professional, teams are going outside of their country. This is the key," Latvian Tocs told reporters. "Europe is also putting in more effort and more money."
The Latvian duo beat the Brazilians 21-16 21-19 in the 43-minute match, thanks to the 36-year-old Plavins' aggressive spikes, attacks and digs, and the 32-year-old Tocs' blocks and defence.
When they won, Tocs lay spread-eagled on the sand court thrashing his arms and legs about in joy. They will face Norway's Christian Sorum and Anders Mol in the semi-finals on Thursday.
Mol and Sorum beat the Russian Olympic Committee's Ilya Leshukov and Konstantin Semenov 21-17 21-19, Mol relentlessly spiking and blocking attacks by the Russian as Sorum went for digs and focused on solid defence in the 43-minute match.
"We made less mistakes. Fighting for the medals in the Olympics is dream come true. We hope to bring some medals back to Norway," Mol told reporters.
