By Julia Love
SAN FRANCISCO (Reuters) - The new iPhone 6s and 6s Plus arrive in stores and at consumers' doorsteps on Friday, kicking off a sales cycle that will be scrutinized for signs of how much juice Apple's marquee product has left.
Apple has a tough act to follow after the success of the iPhone 6, but sales are expected to benefit this year from the inclusion of the Chinese market, where the gadget's debut was delayed in 2014 due to regulatory issues.
With one of Apple's largest markets in the fold, analysts expect the company to sell about 12 million to 13 million phones in the first weekend of availability, up from more than 10 million last year.
Eager consumers from San Francisco to London to Sydney have camped out for days prior to the release, and Apple said earlier this month that based on pre-orders, sales of its new phones were on pace to beat last year's first weekend performance.
"The stage is set for Apple to show year-over-year growth over the Herculean iPhone 6 sales," FBR Capital Markets senior analyst Daniel Ives said.
Apple relies heavily on the sale of its flagship iPhones, which generated nearly two-thirds of its revenue in the latest quarter. First released in 2007, the iPhone is Apple's best-selling device ever.
After a dramatic redesign last year in which Apple enlarged the iPhone's screen and added mobile payments, the iPhone 6s and 6s Plus boast more modest improvements.
The phones, which are the same size as last year's models, feature improved cameras and 3D touch, a display technology that responds according to how hard users press their screens.
Apple executives have said just a fraction of their customers have upgraded to the iPhone 6, suggesting they have plenty of room to grow this year. And lackluster offerings this year from rival smartphone manufacturer Samsung will help Apple's stand out in the marketplace, analyst Patrick Moorhead of Moor Insights & Strategy wrote in an email.
"Over the long haul, the 6s will eclipse the 6 as Apple is even more competitive versus Samsung in emerging regions and is gaining share in traditional regions," Moorhead wrote in an email. "Samsung didn't bring a whole lot of compelling features to consumers with their new lines of phones."
The iPhone 6s and 6s Plus, which start at $199 and $299 with a two-year contract with a mobile service provider, will be available on Friday in Australia, Canada, China, France, Germany, Hong Kong, Japan, New Zealand, Puerto Rico, Singapore, the United Kingdom and the United States.
(Editing by Stephen R. Trousdale and Alan Crosby)