The antitrust lawsuit filed on behalf of 40 customers in a federal court said allowing the merger to proceed would put 90 percent of the domestic air travel business in the hands of four companies, United, Delta, Southwest and the combined American Airlines and US Airways.
The Christian Science Monitor reported Wednesday that both airlines responded quickly.
Mike Trevino, a spokesman for AA said the merger involved two airlines that had very few competing routes.
"As is often the case with high profile mergers, lawsuits containing baseless allegations such as these are filed and they are successfully defended," said US Airways spokesman Ed Stewart.
He also said no airport hubs would be closed due the merger.
The lawsuit was filed by antitrust lawyer Joseph Alioto, who previously filed a similar lawsuit to block the proposed Southwest Airlines acquisition of AirTran Airways.
But that case was filed a day after the deal closed, which was a contributing factor to the end result, which was an order for Alioto's law firm to pay $67,495 of the airlines' legal costs.
The U.S. Justice Department along with 19 states is already reviewing the merger proposal. If it is allowed to proceed, it would make American the world's largest airline, the newspaper said.