An IDC survey said sales of tablets in the March-June period were 49.3 million, up a modest 11 per cent from a year ago but down 1.5 per cent from the previous quarter.
Apple, which popularised tablets with its iPad, remained the largest single vendor but its market share fell to 26.9 per cent from 33 per cent last year.
IDC's survey showed Apple shipped 13.3 million iPads, above Apple's own report this week showing 12.3 million sold.
South Korean Samsung saw essentially flat sales in the period of 8.5 million units, while its market share slipped to 17.2 per cent from 18.8 per cent a year ago.
IDC's Jitesh Ubrani said there has been "growth amongst the smaller vendors and a levelling of shares across more vendors as the market enters a new phase."
"Until recently, Apple, and to a lesser extent Samsung, have been sitting at the top of the market, minimally impacted by the progress from competitors."
The biggest gains came from China's Lenovo, which boosted its market share to 4.9 per cent on 67 per cent sales growth.Taiwan-based Asus, which makes some Google-branded tablets, saw its market share edge up to 4.6 per cent.
"The market is still being impacted by the rise of large-screen smartphones and longer than anticipated ownership cycles," said IDC's Jean Philippe Bouchard.
"We can also attribute the market deceleration to slow commercial adoption of tablets. Despite this trend, we believe that stronger commercial demand for tablets in the second half of 2014 will help the market grow and that we will see more enterprise-specific offerings, as illustrated by the Apple and IBM partnership, come to market."
The research firm Strategy Analytics said in its survey this week that global tablet shipments reached 52.9 million units, up just six per cent from the same period a year ago.