The steep declines in tech on Thursday have left some investors uneasy.
"Given the market's seemingly relentless climb higher on the back of the mega-cap tech names, it should be no surprise that a pullback was in the offing as the market became increasingly extended and overbought", said Quincy Krosby, chief market strategist at Prudential Financial Inc.
Wall Street tumbled on Thursday, with hundreds of billions of dollars wiped off the value of America's booming tech sector amid worries of a Nineties-style bubble.
Big Tech companies have posted outsized gains in recent months as investors bet that they would continue posting huge profits even with many coronavirus restrictions still in place as people spend even more time online with their devices.
The S&P 500 Index retreated from a record high and fell more than 4% in afternoon trading, set for its biggest drop since June amid declines in Apple, Microsoft, Amazon and Facebook. Bitcoin fell as much as 7.6%.More news: AFL grand final to be played in Queensland for first time
The moves are more muted than the 5 per cent plunge on the tech-heavy Nasdaq overnight, or the S&P 500's 3.5 per cent drop, which traders said was overdue given recent frothy gains.
Meanwhile, U.S. health officials have told states to prepare for a Covid-19 vaccine to be ready by November 1, an aggressive goal that suggests availability just before the presidential election.
Sentiment wasn't helped by a warning from United States infectious diseases expert Dr Anthony Fauci who said there is doubt a Covid-19 vaccine will be developed by the end of October.
Elsewhere, oil slumped. Major equity gauges in Asia were mixed.
A report by payroll processor ADP, widely watched as a forerunner of government employment data due out Friday, showed the private sector added 428,000 jobs in August, less than half the 1 million expected by forecasters.More news: Netflix shows and movies that you can watch for free
The S&P 500 Index sank 4% as of 3:10 p.m. The energy sector, in contrast, fell just 0.7%, but financials shed 1.6%. Brent crude, the global benchmark, fell 0.8% to $44.07 per barrel.
The MSCI Emerging Market Index dipped 1%.
The euro rose 0.1% to $1.1862. The yen last stood at 106.21 per dollar.
The yield on 10-year Treasuries fell three basis points to 0.62%.
West Texas Intermediate crude declined 0.5% to $41.18 a barrel.Gold was at $1,931.08 an ounce, little changed.More news: Serena Williams to face Margarita Gasparyan in the US Open
Gold lost ground with the December futures contract falling $US6.90, or almost 0.4%, to settle at $US1,937.80 an ounce.