The doubles match is scheduled for later Saturday.
The Argentines' second win against the United States in 11 meetings in the competition gave them their first title since 1997. The Americans were seeking their fifth title and first since 1993.
Querrey took the first set in 24 minutes after breaking in the opening game.
Monaco responded by breaking to start the second set, saving a break point in the next game and then breaking again to lead 3-0.
After exchanging breaks early in the final set, Monaco broke again to lead 4-3. He then saved five break points to hold for 5-3 before a further break gave him victory.
Zeballos failed to convert a match point in the second set. In the decider, he broke for the first time to close out the match.