Russia and Denmark both need just one point from three rubbers to retain their place in Davis Cup by BNP Paribas Europe/Africa Zone Group I next year.
The Russians took a 2-0 lead into the second day in Moscow after Karen Khachanov, who set a record by becoming the youngest Russian to compete in Davis Cup, recorded a straight sets win over Dean O'Brien.
The home team doubled the advantage after Alex Bogomolov Jr overcame Jacob de Klerk from a set down, winning 67(3) 60 64 61 to put the Russians in the driving seat at the Olympic Stadium.
In the battle of the two Scandinavian nations it was Denmark who took a surprising 2-0 lead over Sweden after Frederik Nielsen defeated Swedish junior Elias Ymer 76(6) 62 46 75 and Martin Pedersen battled past Markus Eriksson 67(7) 63 76(3) 64.
The two teams that win this weekend will maintain their place in Europe/Africa Zone Group I, while the losers will be relegated to Europe/Africa Zone Group II next term.
For live scores from Russia v South Africa click here