SAN FRANCISCO – Apple Inc. is under the gun to produce a big retail launch for its new iPhone models this weekend, as more investors have begun questioning the company's strategy with its most important product.

Apple is putting the iPhone 5S and 5C models on sale at its retail stores and website Friday morning. It will be the company's first time launching two new iPhones simultaneously — though the iPhone 5C is largely the same as last year's iPhone 5 with a plastic casing. The company started taking preorders for the 5C model on Sept. 13.

Early signs have emerged of strong demand — at least relative to supply — for the iPhone 5S. The device reportedly sold out of pre-sale stock in China, though it was not available for sale in the U.S.

This launch poses higher stakes for the company, given the cloud that has grown around Apple since its last iPhone debuted. In that time, the company's sales growth has stalled, earnings have begun to creep down and the stock — which topped the $700 mark for an all-time high the week the iPhone 5 went on sale — has shed about one-third of its value.

The iPhone represents more than half of Apple's total revenue base, as well as a majority of its profits, according to most estimates. Most of Wall Street's ratings and targets on the company are keyed closely on the outlook for the iPhone business.

Analysts are looking for the company to exceed last year's record, when Apple said iPhone 5 sales topped 5 million units over the opening weekend. That's because this year's launch not only includes two iPhone models, but it's also the first time that China will be included in the retail debut.

"If not for China, we'd be at a range of 4 [million] to 5 million," Gene Munster of Piper Jaffray & Co. said. He is expecting that Apple will sell between 5 million and 6 million total iPhone units over the launch weekend.