Apple's ascent came as the overall U.S. market rose much less than expected, with shipments rising only 3.8% from the same period a year ago to 18.9 million units, IDC said Wednesday. The research firm had projected an 11% increase year over year.
However, Apple outpaced the overall market and the other top five U.S. vendors, shipping 24.1% more Macs, or 2 million units. Apple ended the quarter with a 10.6% share, rising above Acer, which fell to fourth place with 10.3%.
Apple rode the popularity of the iPad in selling more Macs. At the same time, the iPad drew sales away from netbooks. Shipments of the inexpensive mini-laptops continued to slow in the third quarter, IDC said.
"Apple's influence on the PC market continues to grow, particularly in the U.S., as the company's iPad has had some negative impact on the mini-notebook market," IDC analyst Bob O'Donnell said in a statement. "But, the halo effect of the device also helped propel Mac sales and moved the company into the number three position in the U.S. market."
Overall, slower-than-expected back-to-school sales contributed to the slow growth in the U.S. market. Hewlett-Packard continued to lead the market with shipments rising 2.7% to capture a 24.3% share. Dell remained at No. 2, but shipments slipped 4.9% to end the quarter with a 23.1% share.
Rounding out the top five in the United States was Toshiba, which saw shipments rise year-over-year by 11.6% to capture an 8.4% share.
Worldwide, the PC market grew 11% from a year ago to 89.7 million units, IDC said. The increase fell short of expectations by 3 percentage points. The slowdown was due to consumers spending less than expected, IDC said. Businesses, however, met expectations, as companies continued to toss aging systems that were held longer than normal because of last year's economic recession.
PC makers in September saw a "good rally" in consumer spending that could set the stage for a solid fourth quarter. Sales drivers could include budding excitement over new media-centric tablets expected to hit the market to take on the iPad during the holiday shopping season, as well as continued buying by businesses, IDC said.
The global PC market saw some changes in vendor rankings in the third quarter. While HP remained the world's largest PC maker, Dell once again slipped to third behind Acer, despite a 9.7% growth rate year-over-year compared to Acer's 7%, IDC said. Rounding out the top five were fourth-place Lenovo, followed by Asus. However, Lenovo had the highest growth year-over-year of the top five at 32.9%, followed by Asus, 30.5%.