Their surge – both have gained by a third this year – has made them the world’s best performing large-cap tech stocks and highlights how these nimble Asian firms are thriving while rivals Apple (AAPL) and Alibaba (BABA) have struggled.

“These companies can grow earnings despite weaker global growth,” said Andrew Gillan, head of Asia ex-Japan equities at fund managing firm Henderson Global Investors, which is overweight on Asian technology firms.

“The operating fundamentals of the Chinese internet sector particularly have surprised positively in the most recent quarterly results.”

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While many investors remain upbeat about Samsung and Tencent, some caution the firms are vulnerable to rapid swings in sentiment on any sign of slowing momentum. Samsung and Tencent have been more volatile than the Asia tech sector and the broader market this year.

On Wednesday, Samsung said sales of its latest flagship smartphone were out-stripping supply, but second-half profits could still take a hit if production shortfalls are not fixed and a recovery in components demand fails to eventuate.

Moody’s Investor Service also warned that Samsung’s profit margins might narrow in the second half because of seasonal factors in the consumer electronics business and competitive pressures.

For Tencent, the market expectations that are driving shares higher are themselves a risk, according to Nomura. A faster-than-expected slowdown in personal computer game revenue, aggressive spending and new products or business models from competitors could weigh on earnings, the bank warned.

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THE NUMBERS

Samsung and Tencent have added about $30 billion in market value since Thursday, surging to all-time highs. Tencent is valued at $249 billion, only 4% smaller than the most valuable Asian firm, China Mobile, at $259 billion. Samsung is now worth $239 billion.

Tencent is now the world’s 12th-biggest company by market value and Samsung the 17th-largest, Thomson Reuters data shows. That’s up from Nos. 26 and 33 respectively just five months ago, according to a PricewaterhouseCoopers ranking released March 31.

Samsung shares’ have significantly outperformed Apple’s – the Korean firm has leapt 50% over the past year, while the U.S. company has gained 3% amid concern about weak sales in China.

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The gap between Samsung’s price-to-earnings ratio of 12.4 and Apple’s 12.7 is now the narrowest since late 2011, although Samsung is still worth less than half the $586 billion Apple, according to Thomson Reuters data.

Samsung’s share price growth spurt comes after years of struggle in its smartphone business which left investors impatient for higher returns.

The firm revived mobile profits by restructuring its product line-up this year and is seeking ways to sustain earnings momentum. Buybacks and higher dividends have also boosted shares.

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Tencent is significantly more expensive than Samsung. The Chinese internet firm, whose popular WeChat and Weixin messaging apps in China saw active monthly user numbers jump 34% in the second quarter, trades at 46.8 times earnings, closing in on Facebook’s 59.

China’s slowest economic growth in 25 years and some questionable acquisitions have clouded the outlook for Chinese e-commerce giant Alibaba, but Tencent has managed to thrive thanks in part to its focus on rapidly growing mobile gaming.

Tencent outshone peers including Baidu (BIDU) with a forecast-beating 47% jump in second-quarter profit, after it diversified into areas such as music, video and advertising.

HSBC expects further earnings growth, driven by new income streams such as advertising, premium content, cloud services and finance.