Secretary of State George P. Shultz pressed Japan yesterday to open its markets even more to American products "to reduce serious streses" in U.S.-Japanese relations.

"In anticipation of liberalized access to Japanese markets, the Reagan administration, for its part, pledges to continue to oppose protectionist measures in the Congress," continued Shultz in what appeared to be an effort to encourage the government of Prime Minister Yasuhiro Nakasone to make further trade concessions before President Reagan visits Japan in November.

American and Japanese officials have been working quietly during the past month to clear away some of the trade problems between the two countries before the Tokyo visit takes place.

The key problem is the extension by Japan for a fourth year of its voluntary curbs on auto exports to the United States, which American car makers insist is necessary for the industry's economic recovery. That extension, moreover, is seen as critical to administration efforts to fight domestic-content legislation supported by labor and aimed at stopping the sale of Japanese-made cars in the United States.

Shultz underscored the importance of the United States' ties with Japan by driving almost 50 miles from Washington to address the sixth Shimoda Conference in the midst of dealing with the crisis caused by the Soviets shooting down a Korean passanger plane. The conference brought together about 90 Americans and Japanese to Airlie House in Warrenton, Va., to discuss economic and security issues facing the two countries.

In his speech, the secretary of State walked a narrow line between welcoming recent Japanese efforts to reduce trade barriers and urging them to do more. He mentioned, for instance, the $60 billion in two-way trade between the countries but omitted the United States' $12.2 billion deficit last year. That deficit is sure to grow this year; it reached $12.4 billion in the first three months of 1983, according to Commerce Department figures.

He said the administration welcomed moves by Nakasone's government to cut away red tape that stifles access of American products to Japan's markets. But Shultz also pushed the Japanese to go further in liberalizing its trade barriers.

"The strength of Japan's economy clearly allows for further action now with respect to important trade restrictions that remain, such as agricultural import quotas and the de facto limitation of major areas of domestic procurement to domestic firms," Shultz said.

He pointed out that Japan's economy depends on an open world trading system and held out President Reagan's decision in March against imposing trade sanctions against the Japanese machine took industry "as an example of his administration's dedication to maintaining the free trading system."

Nakasone, in a message read at the conference, suggested that the United States ease its pressures on Japan over trade. He said the question of Japanese quotas on imports of American beef and citrus "must be considered against the backdrop of Japan being the biggest and most stable customer of U.S. agriculture products."

Shultz also urged efforts in both Tokyo and Washington to end the disparity of an undervalued yen and overvalued dollar so that "exchange rates more accurately reflect the real comparative advantages of our economies."