BEIJING -- At an event Tuesday morning in San Francisco, Apple is expected to unveil a fifth-generation iPad as well as a follow-up version of the iPad mini it released a year ago.

The new iPads come at a critical moment. Samsung Electronics Co is flooding the market with an array of screen sizes and options. Amazon.com Inc is undercutting Apple on price. Its new Kindle Fire tablets, with faster processors and improved screens, start from $229. Microsoft Corp aims to use its corporate connections to tap a new segment of the tablet market. Google Inc and Sony Corp are pushing ahead with new tablets of their own, while Asustek Computer Inc and Lenovo Group Ltd. are gaining market share in parts of Asia where prices matter more.

Research firm Gartner expects Apple's share of the tablet market to fall below 50% this year, from 65% in 2011. Competitors are starting to squeeze the Mini with larger-screen smartphones. Meanwhile, declining tablet prices further pressure Apple to engage in the type of price competition that it has avoided historically.

According to the Wall Street Journal, tablets are now Apple's second-biggest source of revenue after the iPhone, bringing in nearly $26 billion in the nine months ended June 30. But Apple sold fewer iPads in the three months ended in June than the same period a year earlier, the first time that had happened since the device appeared in 2010.

Apple has said that it priced the Mini aggressively, with smaller profit margins than the company average, because of the long-term potential in the tablet market.

"At the end of the day," an IDC analyst said, "the companies left standing are those who put out good products that people want and make money doing it."