Watch: New York Stock Exchange

The S&P 500 Index opened down 4.2% and NASDAQ 100 fell 5.4% after global markets overnight plunged over investor worry about a possible recession in the world's biggest economy.

Driving the rout: Sharp declines in high-flying tech stocks including Nvidia, Apple, Amazon and Google. As investors ran to buy U.S. treasuries, mortgage rates declined, opening the door for some borrowers to refinance, experts said.