That compares with $3.1bn in sales seen in the first half of last year's event.
Singles' Day is considered the world's biggest online retail sales day. It compares with "Cyber Monday" in the US - the day after Thanksgiving also marketed as a big online shopping day.
Alibaba said it expected to break sales records during the annual event, offering big discounts to boost sales.
"I bet the number [of goods bought] is going to be scary," said Alibaba's executive chairman Jack Ma last week. He estimated that 200 million packages would be shipped from orders made during the day.
Last year, Alibaba shipped more than 150 million packages worth about $5.75bn in gross merchandise volume.
Before midday on Tuesday, sales had already hit $4.9bn, Mr Ma told official state broadcaster CCTV.
Singles' Day in China was adopted by Alibaba in 2009 to boost sales, but dates back to at least 1993, when students at Nanjing University are believed to have chosen the date as an anti-Valentine's Day where single people could buy things for themselves.
Since then, it has gone on to become a massive day of sales for China's fast growing e-commerce market.
The market is expected to grow at an annual rate of 25% over the next few years, from $390bn in 2014 to $718bn in 2017, according to a recent study released by management consulting firm AT Kearney.
Analysis: Celia Hatton, Beijing correspondent, BBC News.
When did an obscure holiday devoted to single people, a kind of anti-Valentine's Day, become China's biggest shopping day of the year? Many in China find the transition baffling.
"November 11 used to be 'Singles' Day'. Then it became the Shopping Day, and now it's Couples' Shopping Day," complains one user on Weibo, China's version of Twitter. "Looking across the globe and over the years, there's never really been a day dedicated to single people."
Others point out that China's biggest online shopping platform, Alibaba, engineered the change.
Alibaba trademarked the term "Double-11", a popular term describing the November 11th holiday, in 2012.
The website earns a significant chunk of its profits on that day.
"They say behind every successful man, there's a woman quietly supporting him. Behind Jack Ma, [Alibaba's founder], there're tens of millions of female shopaholics," jokes one Weibo user.
Frustration lingered in the background of many Weibo forums.
Shoppers complained the online deals were too good, tempting them to spend money they didn't have.
"Please Singles' Day, can you pass by quickly?," reads a typical comment. "I didn't think I needed anything until today when I feel like I need everything I see. I'm so helpless!"