* China's export growth in single digits, misses forecast

* Imports lag, reflecting weak demand

* Trade balance narrows from July's record

* Booming trade momentum expected to weaken

BEIJING, Sept 7 (Reuters) - China's exports and imports lost momentum in August with growth significantly missing forecasts as surging inflation crippled overseas demand and fresh COVID curbs and heatwaves disrupted output, reviving downside risks for the shaky economy.

Exports rose 7.1% in August from a year earlier, slowing from an 18.0% gain in July and marking the first slowdown since April, official data showed on Wednesday, well below analysts' expectations for a 12.8% increase.

Outbound shipments have outperformed other economic drivers this year but now face growing challenges as rising interest rates, inflation and geopolitical tensions pummel external demand.

The disappointing August trade figures rattled global financial markets, which have already been buckling under a surging dollar and the prospect of much higher U.S. interest rates.

"It seems the export softness arrived in earlier than expected, as recent shipping data suggests that demand from the U.S. and EU has already slowed as shipping prices have been falling significantly," said Zhou Hao, chief economist at Guotai Junan International.

He expects pricing effects will continue to disrupt trade and said import growth in real terms had already turned negative since the late first quarter, suggesting more headwinds for demand.

Responding to the disappointing data, China's yuan extended losses, losing 0.36% to 6.98 per dollar and approaching the psychologically crucial 7 mark.

Despite languishing around two-year lows, the weakening yuan has failed to give China's exports the competitive edge they need to make up for softening external demand.

The slower growth is also in part due to unflattering comparisons with strong exports last year, but also worsened by more COVID restrictions as infections spiked and heatwaves disrupted factory output in southwestern areas.

Export hub Yiwu imposed a three-day lockdown in early August to contain a COVID outbreak, disrupting local shipments and delivery of Christmas goods amid the peak season.

Bucking the broader trend, auto exports remained robust in August, jumping 47% from a year earlier, according to Reuters calculations based on customs data.

In the first eight months, China exported 1.9 million units of cars, up 44.5%, supported by strong demand for new energy vehicles in Southeast Asia.

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