Düsseldorf - Argentina advanced to the World Team Cup final by beating the United States on Thursday.
The other finalist will be decided on Friday between Germany, Serbia, Russia and Spain.
Juan Monaco started off Argentina by defeating Mardy Fish 7-6 (4), 7-5, then Juan Ignacio Chela beat John Isner 6-1, 7-6 (1) for an unassailable 2-0 lead in the best-of-three tie.
Fish held opportunities in each set, but Monaco was able to hold off the challenge.
In the opening set, Fish immediately recovered a break to level at 3-3 and then held four set points at 5-4.
In the second set and following four successive breaks of serve, Fish held two break points to lead 4-3 and another to lead 6-5, before Monaco broke for the match and claimed victory on his third match point.
"I had set points in the first set and obviously a lot of chances in the second so it's just a match that didn't go my way," Fish said.
"Playing someone like Monaco on clay is always going to be a tough test. But I've played two singles matches, two doubles matches, and it's great preparation for the French (Open which starts on Sunday)."
Isner never looked comfortable against Chela in the opening set and he faced an uphill task after dropping his serve as early as the third game. Further breaks for 4-1 and 6-1 followed.
Isner offered greater resistance in the second set and he held break point at 3-3 and 4-4, but managed to salvage just one point in the tiebreaker.
The winner of the Blue Group will be decided on Friday, with four teams still in contention.
With a singles and a doubles to be played by each team, Germany and Serbia enter the final day of the round-robin with 1-0 leads.
Philipp Petzschner put Germany ahead with a 6-2, 6-4 win over Russia's Igor Andreev, and Serbia held an advantage over Spain after Viktor Troicki beat Marcel Granollers 6-4, 6-4.
Results from the third day of the ATP Team World Cup on Thursday:
Argentina 2 USA 0
Juan Monaco (ARG) bt Mardy Fish (USA) 7-6 (7/4), 7-5
Juan Ignacio Chela (ARG) bt John Isner (USA) 6-1, 7-6 (7/1)
Kazakhstan 1 Sweden 1
Mikhail Kukushkin (KAZ) bt Christian Lindell (SWE) 6-0, 6-1
Robin Soderling (SWE) bt Andrei Golubev (KAZ) 6-7 (4/7), 7-6 (9/7), 6-3
Serbia 1 Spain 0
Viktor Troicki (SRB) bt Marcel Granollers (ESP) 6-4, 6-4
Germany 1 Russia 0
Philipp Petzschner (GER) bt Igor Andreev (RUS) 6-2, 6-4