* U.S.-China trade talks remain focus for grains markets
* Trade negotiators said nearing phase one deal
* Rain to slow U.S. harvest this weekend, dry beyond that (Recasts with U.S. market open, adds quote, updates prices, changes byline; changes dateline, previous PARIS/SINGAPORE)
CHICAGO, Oct 25 (Reuters) - U.S. corn futures firmed for the first time in three sessions on Friday while wheat rose for just the second time this week on optimism the United States and China are moving closer to a trade deal that could bolster American farm product exports.
Soybeans slumped to a two-week low on an accelerating harvest and as Chinese purchases of U.S. shipments have not lived up to trade expectations after Beijing offered crushers a quota of duty-free imports early this week.
All three markets were on pace for weekly declines.
Grain traders have focused on U.S. Midwest corn and soy harvest weather as farmers have struggled to gather developing crops. Storms are expected to chase farmers from fields in the eastern Corn Belt this weekend ahead of a drier weather pattern.
The market has also remained cautious this week with U.S. and Chinese negotiators working to nail down a phase one trade agreement ahead of an expected meeting next month between presidents Xi Jinping and Donald Trump at a summit in Chile.
Trade officials from both countries are "close to finalizing" some parts of an agreement after high-level telephone discussions on Friday, the U.S. Trade Representative's office said. Accelerated U.S. agricultural product purchases are among measures being discussed.
"Based on comments from both the U.S. and China, we're going to have the beginnings of a deal at the APEC conference," said Jim Gerlach, president of A/C Trading.
"Demand is what's been holding the market down so whenever you get good news on demand it takes a lot of pressure off the market."
Chicago Board of Trade (CBOT) December corn futures were up 1-1/2 cents at $3.88-1/4 a bushel at 12:19 p.m. CDT (1719 GMT), but still on pace for a second straight weekly decline.
CBOT December wheat was up 2 cents at $5.18 a bushel but down more than 2% in the week, which would be its first weekly drop in two months.
CBOT November soybeans fell 6 cents to $9.27-1/4 a bushel, the lowest since Oct. 11. Soybeans were positioned for their second straight weekly drop after hitting a four-month high at mid-month.
Argentina's presidential election on Sunday is being monitored to see if it changes export tax policy in the major wheat, corn and soybean supplier in the midst of an economic crisis. (Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Rashmi Aich, Susan Fenton and Tom Brown)