Chinese President Xi Jinping and U.S. President Donald Trump attend a welcome ceremony at the Great Hall of the People in Beijing on November 9, 2017.

Even as trade tensions continued to heat up, President Donald Trump would make sure to refer to China's president, Xi Jinping, as his "friend." On Friday, though, Trump unveiled a new label for his Chinese counterpart: "enemy."

In one of a series of tweets that rattled markets, the president posed a question to his more than 60 million followers comparing Xi to Federal Reserve Chairman Jerome Powell.

"My only question is, who is our bigger enemy, Jay Powell or Chairman Xi?" Trump wrote.

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The tweet came shortly after China announced that it will impose 5%-10% tariffs on $75 billion worth of U.S. goods and reinstate duties on American autos. The tariffs will come in two batches, on Sept. 1 and Dec. 15, which are the same days that Trump's newest round of tariffs on Chinese goods will go into effect.

The S&P 500 index of large publicly traded companies was down about 1.8% Friday morning after briefly going positive. Trump also said Friday that he had "hereby ordered" U.S. firms to seek an "alternative" to China.

At first blush, Trump's comment was striking not for its slam on the communist leader, but for the critique of the American central bank chairman whom Trump himself appointed.

But it also suggests that the president's personal relationship with Xi, which Trump has touted as the best route to completing a major trade deal uniting the world's two largest economies, is at a low point.