US index futures were tumbling early Wednesday, following global stocks lower after manufacturing data out of China signaled a slowdown.

The S&P 500 fell 6.2% in 2018, booking its worst year since the financial crisis and worst December since the Great Depression.

Read more at Markets Insider.

US stocks look set to tumble after soft data from China cast a somber mood on global markets on the first trading day of the New Year.

In China, the Caixin/Markit Manufacturing Purchasing Managers' Index for December came in at 49.7. The survey most recently dropped under 50 — the mark of a contracting sector — back in May 2017.

"This is not a good indicator as we eye tariffs biting even harder in 2019 than they did last year," Neil Wilson, the chief market analyst for Markets.com, said.

Read more: Stop blaming the machines. Algorithms aren't causing the wild stock market swings.

It's a brutal start to the new year after 2018 ended on a sour note for markets. The S&P 500 was down 6.2% in 2018, booking its worst year since the financial crisis and worst December since the Great Depression.

In addition to the weaker Chinese data, factory output was seen falling across Asia last month.

"An increasing amount of data is pointing to the Chinese economy losing steam and with new orders falling for the first time in 2 1/2 years, the outlook doesn't look great either," Jasper Lawler, the head of research at London Capital Group, said.

Here's the roundup: