Coca-Cola maintained the top-spot, but sales of its sugar-free alternative fell by 6.6%, according to new figures by trade publication Beverage Digest.
Diet Pepsi sales fell by more than 5%.
Nonetheless, both companies posted a modest increase in overall sales in 2014.
Once a common sight on dining tables across the country, fizzy drinks have fallen out of favour in the US over the past decade.
Low-calorie alternatives that replace sugar with sweeteners such as aspartame have also declined in popularity, due to concerns about the side-effects of their ingredients.
Consequently, sales of healthier alternatives have risen sharply. Bottled water flew off the shelves in 2014, boosting Nestle's sales by 9%.
Energy drinks and cold coffees also increased in popularity.
"There has been a tremendous migration to other categories in recent years," John Sicher, editor of Beverage Digest, told the BBC.
Concerns by some consumers about "obesity and health and wellness" have led to greater scrutiny of the ingredients used by big brands, he added.
The likes of Coca-Cola and Pepsi are now trying to get "calorie levels lower with the use of natural sweeteners".
Coke recently released Coca-Cola Life, which is made with stevia sweetener and has 27 calories per 100mL, compared with 42 for regular Coke.
The company's overall sales rose last year for the first time since 2000.
Despite the decline in fizzy drink sales, the overall refreshment beverage market in the US grew by 2.2% - the strongest rate for several years, according to New York's Beverage Marketing Corporation.
The consultancy added that the US drinks business was worth $160bn in 2014.

Global prices have fallen by more than 60% since 2014.
The New Zealand-based company said in a  statement that the employment cuts were to result in savings of up to NZ$60m ($39m; Â£25m) per year.
The company said the measures were needed to remain strongly competitive in today's global dairy market.
"Reducing the number of roles in our business isn't about individual competency; it is about continually improving the way we deliver performance," said Fonterra chief executive Theo Spierings in a statement.
The layoffs are part of an ongoing business review and will come at a one off cost of NZ$12m - $15m, the company said.
Mr Spierings did not give a breakdown of which part of the business the job losses would be in.
Consultations on further restructuring measures were announced for August.
The dairy co-operative is the world's largest milk exporter and Fonterra products are popular across Asia, particularly in China.
The company's profits have been falling for nearly two years in the face of volatile dairy prices.
Shares dropped by 1% in Thursday trade.
Pepsi outsold Diet Coke last year to become the second most popular carbonated drink in the US, as health-conscious consumers continued to shun products with artificial sweeteners.
[NEXT_CONCEPT]
The world's largest dairy producer, Fonterra, said it plans to cut 523 jobs in the face of tumbling dairy prices.