(Reuters) -Shares of chip company Advanced Micro Devices and server maker Super Micro Computer tumbled in premarket trading on Wednesday after the companies posted underwhelming results in their data center segments, shaking investor confidence in their AI growth narratives.

AMD dropped nearly 5%, while Super Micro plummeted about 17%. The selloff comes as chipmakers face mounting headwinds, including weak demand, rising inventories, and the threat of new U.S. tariffs on semiconductor imports, expected as early as next week.

Analysts say that given the strength of the overall server market, Super Micro's disappointing results likely reflect market share losses to larger rivals.

SMCI peer Dell down 3.1%.

CHINA DRAG, MI355 RAMP IN SPOTLIGHT

AMD's data center unit, which includes its Instinct AI chips and server processors (CPUs), posted a 14% revenue rise to $3.2 billion in the second quarter, slightly below market expectations.

"AI outlook did not show the sort of upside some investors were looking for," said Jefferies analysts.

In contrast, Nvidia's data center segment jumped 73% to $39.11 billion in its fiscal first quarter.

In a post-earnings conference call, AMD CEO Lisa Su said the company's AI chip revenue declined year-over-year because of the U.S. restrictions on export of its MI308 chips to China.

The company noted progress on export license reviews for the chips, but gave no clear timeline for lifting the restrictions.

"Potential addback of MI308 revenues from lifting of export restrictions also appears to be more muted than market expectations," HSBC added.

SUPER MICRO: MARGIN SQUEEZE, GUIDANCE MISS

Super Micro missed fourth-quarter estimates as it competes with larger rivals in AI server hardware, with execution missteps and Nvidia chip delays weighing on its performance.

Analysts say any weakness in the high-expectation AI space tends to spark sharp investor backlash.

"We expect better chip availability this fiscal year compared to the last two quarters, which will help us grow better," Super Micro CEO Charles Liang said on the post-earnings call.

J.P.Morgan analysts noted that AI remains the "preeminent driver of sales growth," with over 70% of fourth-quarter revenue tied to AI platforms. They added that Super Micro's rapid integration of next-gen technologies could support margin recovery once supply stabilizes.

AMD commands a higher forward price-to-earnings multiple of 32.39, compared to SMCI that trades at a more modest 19.69.

(Reporting by Rashika Singh in Bengaluru; Editing by Shinjini Ganguli)