BELGRADE, Feb. 3 (Xinhua) -- The United States gained a third decisive point in doubles match on Sunday, ensuring a Davis Cup triumph as guests against Serbia at the Cair sports center in Nis in the first round of the World Group of the competition.

Serbia, 7th in the Davis Cup rankings, lost to the 9th team, mostly because of the fact that its four best players - Novak Djokovic, Janko Tipsarevic, Viktor Troicki and Filip Krajinovic did not play due to injuries or lack of training.

This left coach Nenad Zimonjic with much a less experienced team.

In the first rubber played on Friday, Djere Laslo from Serbia lost to Sam Querrey of the United States by 1-3 (6-7, 2-6, 5-7, 4-6), while John Isner from the United States triumphed over Serbia's Dusan Lajovic 6-4, 6-7, 6-3, 7-6 in the second rubber.

The doubles match on Saturday was played between Nikola Milojevic and Miljan Zekic from Serbia and Ryan Harrison and Steve Johnson representing the U.S. The 3-hour-match ended in favour of the U.S. team by 3-1 (6-7, 6-2, 7-5, 6-4).

On Sunday one or two single matches are left to be played, but their result will not have any influence on the outcome.

In the quarterfinals, the United States will play against the winner between Belgium and Hungary.?