"Growth in China is expected to be so rapid that the country will nearly catch up to the US in sales by the end of the forecast period," said Corey McNair, author of eMarketer's latest report "Global Digital Travel Sales 2018: Ease of Mobile Payment Options Boost Online Bookings."

"Between 2017 and 2022, China’s growth rate will more than triple that of the US each year. The US will still grow steadily and generate more than $200 billion in annual sales through the end of the forecast period. However, growth is slowing due to the maturity of the digital travel market," he said.

In China, millennials are the largest group booking digital travel fare. These travelers are finding deals via social media and are less reliant on group tours for sightseeing. Since China has a large audience that has yet to book online, there is more room for growth.

India is another major market that will see digital travel sales grow by at least 20.0%. Like China, India benefits from a large number of millennials with a desire to travel. Improving mobile internet connections and adoption of digital payment services like Paytm have helped more of India's consumers access travel opportunities. These developments also enable secure online booking for travelers in India.

Digital travel sales in India will rise 20.0%, or nearly double worldwide growth, to $23.71 billion.