Nalbandian topped Viktor Troicki 6-4, 4-6, 6-2, 6-3 and del Potro defeated Janko Tipsarevic 7-5, 6-3, 6-4.
Troicki was a late substitute for world No. 1 Novak Djokovic, who was to have played the first singles match Friday but pleaded jet lag. His availability for Sunday's singles play is unknown.
Djokovic was key in helping Serbia win its first Davis Cup championship last year, but now the Serbs are one loss from elimination.
To keep that from happening, the Serbs must win Saturday's doubles in which Tipsarevic and Nenad Zimonjic are scheduled to play Argentina's Juan Ignacio Chela and Juan Monaco. If the Serbs win that match, the series will be decided Sunday in singles when Troicki -- or Djokovic -- face del Potro and Tipsarevic plays Nalbandian.
Nalbandian overcame seven double faults in winning two-thirds of the points on serve Friday. He saved nine of the 12 break points Troicki managed and lost his serve once after the second set.
Del Potro dropped just 16 points over 16 service games and had one break in each set in the win. He had 24 unforced errors against 42 winners.
The winner of the series heads to the Dec. 2-4 Davis Cup finals against France or Spain. Spain owns a 2-0 lead in that semifinal.