* Singles' Day trumps Black Friday, Cyber Monday combined
* Alibaba saw Singles' Day sales last year of $17.7 bln
* Growth likely to slow this year - analysts
* More use of technology, but fewer deals expected
SHANGHAI, Nov 10 (Reuters) - Singles' Day, once a celebration for China's lonely hearts and now the world's biggest shopping spree, opens at midnight on Friday, with millions of shoppers eager to snap up cut-price deals.
At a star-studded event in Shanghai, e-commerce giant Alibaba will count down the clock for the annual Nov. 11 event - a 24-hour extravaganza that outsells the U.S. Black Friday and Cyber Monday sales combined, and acts as a barometer for China's consumer spending.
Alibaba alone saw $17.7 billion of goods sold on its platform at last year's Singles' Day.
Around China, consumers have already started scouting for deals and filling up online shopping carts, while delivery men - and robots - are gearing up for the estimated 1.5 billion parcels expected over the next six days.
"This is a big event for China, for the Chinese economy," co-founder and Alibaba vice chairman Joseph Tsai said ahead of the sales bonanza. "On Singles' Day, shopping is a sport, its entertainment."
"There are today over 300 million middle-class consumers in China. Every year, their disposable income increases, they are demanding better quality goods and want a better standard of living. This powerful group is propelling the consumption of China," Tsai said.
Analysts and investors will closely watch the headline sales number, which will race past $1 billion within a few minutes past midnight, and is likely to top last year's total. Spending rose by nearly a third at 2016's sale - the eighth iteration of the event - but that was slower than the 60 percent increase logged in 2015.
At Alibaba's Friday night gala, co-founder and chairman Jack Ma will host guests including actress Nicole Kidman, basketball star LeBron James, and Chinese musicians and film stars such as Zhang Ziyi and Fan Bingbing.
The excitement around the shopping blitz, however, masks the challenges facing China's online retailers such as Alibaba and JD.com Inc, which are having to spend more as they compete for shoppers in a broader economy where growth is slowing.
"A lot of the lower hanging fruit has been picked and there's increased competition for a share of consumer spending," said Matthew Crabbe, Asia Pacific research director at Mintel.
He estimated Alibaba's Singles' Day sales growth would likely slow to around 20 percent. Online retailers were being forced to push offline as well as overseas to attract new shoppers, and the overall online retail market was close to "saturation."
"They're having to spill over out of the purely online realm into the wider consumer market," Crabbe said.
This has sparked deals to buy bricks-and-mortar stores in China, and overseas tie-ups especially in Southeast Asia. Technology, too, has been key, with virtual reality dressing rooms and live fashion shows to attract shoppers to haute couture.
Ben Cavender, Shanghai-based principal at China Market Research Group, noted that brands were being more careful with the deals they have on offer this year to avoid "margins getting killed," and were often asking for deposits in advance.
In previous years, most prices were often halved.
"I think prices seem high and I'm totally lost as to the rules (about discounts)," said Gao Wantong, 21, a student in Beijing, though he said he still plans to stay up until midnight.
Fu Wenyue, a 23-year-old dresser in Shanghai, said she had already spent around 4,000 yuan ($602.54)on clothes, cosmetics and kitchen utensils in pre-event sales, transactions ahead of the day which officially only go through once midnight strikes.
"I've bought some stuff already, but I haven't finished shopping yet," she said.
($1 = 6.6386 Chinese yuan renminbi)
(Reporting by Adam Jourdan and SHANGHAI newsroom; Editing by Ian Geoghegan)