Ruben Bemelmans and Joris De Loore defeated German brothers Mischa and Alexander Zverev 6-3 7-6 (7-4) 4-6 4-6 6-3 to give Belgium a 2-1 lead in their Davis Cup first-round tie.
The first brothers to play together for Germany in the competition saved 15 of the 18 break points they faced but were unable to prevent Bemelmans and De Loore winning in three hours and 13 minutes.
The 19-year-old Alexander Zverev, who levelled the tie 1-1 on Friday, was nominated alongside his older brother in place of Jan-Lennard Struff, who has a stomach bug.
Germany needs to win both of Sunday's reverse singles to preserve its unbeaten record against Belgium.
The younger Zverev was due to play Steve Darcis before German No. 1 Philipp Kohlschreiber faces Arthur De Greef.
