The Institute for Supply Management said Monday that its service-sector index fell to 53 last month, down from 53.9 in November. Any reading above 50 indicates expansion.
A measure of new orders plummeted 7 points to 49.4, the first time it has dropped below 50 since July 2009. A gauge of business stockpiles also fell sharply.
But a gauge of hiring increased 3.3 points to 55.8, evidence that services firms are adding more jobs.
The survey covers businesses that employ 90 percent of the workforce, including retail, construction, health care and financial services firms.
Anthony Nieves, chairman of the ISM's services survey, said the declines in orders and business stockpiles likely occurred after "a little bit of excess" had built up in anticipation of the winter holidays. He expects the orders index to recover in the coming months.
The rise in hiring suggests companies are still "confident enough in the pipeline to add jobs," he added.
Service companies have grown at a modest pace this year. The index briefly rose to an eight-year high of 58.6 in August but then dropped to 54.4 in September. It has averaged 54.7 over the past 12 months.
The survey typically tracks growth in consumer spending, which drives 70 percent of economic growth.
Spending by consumers was restrained for most of last year. But Americans have opened their wallets a bit more in recent months. That could push the index higher in 2014.
Retail sales posted healthy gains in November and December, according to government figures. The biggest increases were at auto dealers, furniture stores, and at electronics and appliances stores. Those outlets have benefited as Americans have purchases more homes and cars. Online shopping has also grown at a strong pace.
However, traditional retailers, such as department stores and clothing stores, haven't benefited as much. Many were disappointed by this season's holiday sales.
The dip in the services index comes after the ISM said Thursday that its manufacturing index remained near a 2 1/2-year high in November. Measures of new orders and employment grew at a faster pace, pointing to higher output in the coming months.
The economy expanded at a 4.1 percent annual pace in the July-September quarter, buoyed by a big increase in companies' stockpiles. That was the fastest pace in nearly two years. Growth was likely slower in the October-December quarter because inventory building probably didn't boost growth by as much.
But economists are getting more optimistic about the fourth quarter. Many now expect growth will reach 2.5 percent to 3 percent at an annual rate, up from around 2 percent just a few weeks ago.