SHANGHAI, Nov 12 (Reuters) - China"s yuan weakened on Friday against the greenback, which has strengthened on expectations of further U.S. monetary tightening, but strong dollar selling by Chinese exporters limited the yuan"s fall. The yuan opened at 6.3950 per dollar and was changing hands at 6.3960 at midday, 60 pips weaker than the previous late session close, despite the People"s Bank of China setting a stronger midpoint rate at the open. The dollar index, which measures the value of the greenback against a basket of major world currencies, hit a three-month high on Friday. The U.S. currency has been bolstered by bets the Federal Reserve will tighten monetary policy further - including raising interest rates next year - to tame inflation which could be stickier the Fed expected. But China"s widening trade surplus, and thus robust demand for exporters to sell the dollar for the yuan, is underpinning the Chinese currency, traders say. "There"s a flood of dollar-selling (from clients)," said a trader at a Chinese bank, adding there are signs major state banks are buying dollars to limit the yuan"s rise. "China"s trade surplus is exerting appreciation pressure on the yuan," said the trader, who is not authorised to speak to the media. Yuan"s fall is also limited by optimism of improving Sino-U.S. ties. U.S. President Joe Biden and Chinese leader Xi Jinping are expected to hold a virtual summit on Monday, sources said, amid tensions over trade, human rights and military activities. China"s official yuan index, which measures the value of the Chinese currency against a basket of currencies, climbed to a nearly six-year high on Friday, having gained more than 6% this year. The yuan market at 0457 GMT: ONSHORE SPOT: Item Current Previous Change PBOC midpoint 6.4065 6.4145 0.12% Spot yuan 6.396 6.39 -0.09% Divergence from -0.16% midpoint* Spot change YTD 2.07% Spot change since 2005 29.40% revaluation Key indexes: Item Current Previous Change Thomson 101.2 100.92 0.3 Reuters/HKEX CNH index Dollar index 95.23 95.163 0.1 *Divergence of the dollar/yuan exchange rate. Negative number indicates that spot yuan is trading stronger than the midpoint. The People"s Bank of China (PBOC) allows the exchange rate to rise or fall 2% from official midpoint rate it sets each morning. OFFSHORE CNH MARKET Instrument Current Difference from onshore Offshore spot yuan 6.3915 0.07% * Offshore 6.5612 -2.36% non-deliverable forwards ** *Premium for offshore spot over onshore **Figure reflects difference from PBOC"s official midpoint, since non-deliverable forwards are settled against the midpoint. . (Reporting by Shanghai Newsroom; Editing by Lincoln Feast)