The re-election of Taiwan President Tsai Ing-wen, whose party favors independence from China, comes at a time when Japan has recently pledged to open a "new era" of ties with the Communist-ruled mainland.

Following the outcome of Saturday's presidential election, Tsai of the Democratic Progressive Party is expected to push policies countering the "one country, two systems" framework that Beijing is threatening to impose on the self-governing island.

But fears are growing in Taipei that Japan, a key trading partner of free and democratic Taiwan, might agree with Beijing on a new political document effectively expressing its support for China's strategy toward what it regards as a wayward province.

As Chinese President Xi Jinping is keen to attain Beijing's goal of cross-strait reunification, the leadership "will definitely take a tougher stance against Tsai's government to thwart its attempt to curb the mainland's influence in Taiwan," a diplomatic source said.

Tokyo's approval of such a document could "mean that Japan would accept the mainland's insistence that Taiwan is a part of China. That would threaten Taiwan and poison its nongovernmental, practical relations with Japan," the source added.

Taiwan and mainland China have been governed separately since they split in the wake of a civil war in 1949. Beijing has since then endeavored to undermine Taipei's quest for international recognition.

While Tokyo severed diplomatic ties with Taipei and established them with Beijing in 1972, Taiwan and Japan have continued to deepen economic cooperation at the initiative of the private sector. Taiwan is now Japan's fourth-largest trading partner.

Despite Japan exercising "voluntary restraint" toward relations with Taiwan in the political arena, "the grassroots relationship between the two is very good," said Tai Wan-chin, a professor emeritus at Tamkang University in New Taipei City.

For decades, Taiwan and Japan have expanded people-to-people exchanges in such fields as business, technology, culture and medical care, he said.

Japanese Prime Minister Shinzo Abe, meanwhile, has striven to improve relations with China, often frayed over wartime history and territorial issues. Beijing for its part seeks better economic ties with Tokyo amid an ongoing trade dispute with the United States.

At talks in Beijing late last month, Abe told Xi, "We want to build a relationship that is suitable for a new era for Japan and China," while the Chinese president said he is willing to raise "Sino-Japanese ties to a new level."

Sources close to the matter said Abe and Xi agreed at the December meeting that Tokyo and Beijing will craft a new political document that will lay the foundation for their future relations -- the fifth of its kind since they normalized diplomatic ties.

The new document is expected to be unveiled when Xi visits Japan in the spring as a state guest for the first time since he came to power in 2013, the sources said.

Japan and China have developed their relations on the basis of the principles of the existing four political documents, respectively signed in 1972, 1978, 1998 and 2008. The latest one stipulates that the two nations would advance strategic and mutually beneficial ties.

At a news conference during his stay in China on Dec. 24, Abe indicated his readiness to hammer out the fifth document, saying Japan and China "will step up efforts to bring results in each area."

A person familiar with the Japanese government's thinking said, "The Foreign Ministry was reluctant to make a new document given the current situation in the region," referring to Hong Kong's months-long protests, China's increasing naval assertiveness and Taiwan.

Abe, however, is "eager to make the fifth document as he has so far failed to secure major diplomatic achievements, although he became Japan's longest-serving prime minister. China has provided him with a window of opportunity," he said.

The person added that it was Beijing which broached the creation of the fifth political document, thus sparking speculation in Taiwan that China will push for the inclusion of its hardline stance on cross-strait relations.

Late last year, former Taiwan President Chen Shui-bian said his Japanese friend had told him that the fifth political document will incorporate Tokyo's support for the policy of China's one country, two systems in Hong Kong.

If that is the case, Taiwan's interests would be seriously damaged, as China would be able to cite the new document in asking the Japanese government to adjust its relations with the island in the future, Chen was quoted as saying by the media.

Over the past few years, Tsai said it has become clear that China's intimidation is aimed at forcing Taiwan to compromise on sovereignty, including Xi's recent proposal to explore a "Taiwanese version of one country, two systems."

"It is impossible for Taiwan to accept the arrangement of one country, two systems," Tsai said in her New Year's address on Jan. 1, adding that the Hong Kong model of the framework is a proven failure as seen by the city's prolonged unrest.

Xi's Communist Party tried to thwart Tsai's re-election by "every available means," said Tatsuhiko Yoshizaki, an expert on foreign affairs issues at the Sojitz Research Institute in Tokyo.

Given its failure to stop her, China is set to "take stronger measures" against Taiwan, he added.

Another diplomatic source said, "Under such circumstances, China may use Japan to get an excuse to press ahead with its reunification bid. Prime Minister Abe should be very cautious about endorsing the fifth political document."