Singles' Day -- named for the repeated digit 1 in the date November 11 -- was created by Alibaba in 2009.
Now in just 24 hours it surpasses the main US online spending spree, the five days from Thanksgiving to Cyber Monday.
Shortly after noon, gross merchandise volume -- Alibaba's key measure of online sales -- had reached 82.4 billion yuan ($12.1 billion), the firm said, already closing in on last year's total.
It had overtaken the 2014 figure by 6.54am, the firm said, with rival e-commerce giant JD.com, which focuses more on electronics, similarly receiving more orders by 9.14am than it had done on the whole of Singles Day 2014.
In the US, total online sales for the five days from Thanksgiving through Cyber Monday last year stood at $11.1 billion, according to Adobe Digital Index.
Alibaba's New York-listed stock has been buoyant this year in spite of worries over slowing growth in China, as consumer spending has expanded while the old industrial economy struggles due to overcapacity and sluggish demand.
China's economy expanded at its slowest rate in a quarter of a century last year and has eased further this year, as Beijing tries to shift growth drivers from investment and exports to domestic consumption.
Alibaba kicked off this year's event with a gala in the southern city of Shenzhen on Thursday night, inviting international and domestic stars from basketball player Kobe Bryant to pop rock band OneRepublic.
Singer Katy Perry was also originally scheduled to perform but cancelled at the last minute due to a family emergency, she said on China's Twitter-like weibo.
Analysts said the event, which has gained more and more traction in the past few years, is key to Alibaba.
The day's turnover was only "a very small fraction of its total annual sales", independent e-commerce analyst Li Chengdong said, but was "a confidence index for the firm".
"If they do well in this, it is a boost to investors' confidence," he told AFP.
But sales growth may not match last year's 60 percent rise, he warned, in the face of economic worries and stricter regulation.
China's commerce watchdog SAIC summoned Alibaba and 14 other e-commerce companies earlier this week, banning them from making up fake orders and lifting prices before giving out discounts.
Alibaba itself has sought to downplay the importance of the turnover statistic.
"I don't have any requirement for the sales figure," Alibaba quoted Chairman Jack Ma as saying in a statement on weibo, adding the day was all about "happiness and joy".
Online shopping is a bright spot in Beijing's efforts to transform the economy, growing 26.1 percent in the January to September period according to the National Bureau of Statistics.
That outstripped both overall retail growth of 10.4 percent in the period, and official GDP expansion of 6.7 percent in each of the first three quarters.
China's State Post Bureau has estimated the country will handle more than one billion packages during the six days from Friday, up 35 percent from last year, the official news agency Xinhua reported.
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