The two aquatic mammals -- Mitya and Solnyshko -- in the Yaroslavl dolphinarium were thrown round discs depicting the flags of the two countries in their pool and had to bring them back.
Both came back at the same time for a 1-1 draw after the first run before the Russian flag was returned faster by Mitya in the last two throws for a predicted 3-1 Russia victory on Saturday.
After trouncing Saudi Arabia 5-0 in the tournament's opening game and then beating Egypt 3-1, Russia lost 3-0 to South Americans Uruguay but still advanced past the group stage.
They then knocked out 2010 World Cup winners Spain on penalties in the round of 16 to set up their quarter-final match against the much-fancied Croats.
The dolphins are part of a growing menagerie of animals predicting World Cup games.
This cornucopia so far includes Samara's white goat Zabiyaka, St Petersburg's cat Achilles, Spartak the lemur in Yekaterinburg, Kaliningrad's hippos Milya, Glyasik and Nizhny Novgorod's tapir Cleopatra as well as Sochi's Harry the otter, who had wrongly predicted Spain would beat Russia.
(Reporting by Reuters Television, Writing by Karolos Grohmann; Editing by Christian Radnedge)
