Also looming large amid dozens of bilateral meetings in Buenos Aires: the gruesome slaying of dissident Saudi journalist Jamal Khashoggi in Saudi Arabia's Istanbul consulate and how the Saudi crown prince, who is alleged to have ordered the killing, is received by world leaders.
The leaders - 35 men and two women - posed for the "family photograph" traditionally taken at G20 gatherings, before heading to the plenary chamber room for initial round-table discussions, to be followed by a series of one-to-one conversations between leaders in the center's maze of meeting rooms.
The group of 20 accounts for 85% of the world's GDP and two-thirds of its population.
The two countries are also at odds militarily over China's extensive claims in the South China Sea and US warship movements through the highly sensitive Taiwan Strait.
"Five years ago, when I first attended the G20 leaders' summit, I called for the maintenance and development of an open world economy", Xi said, according to a transcript of his speech.
Xi and other leaders from the so-called BRICS group of leading emerging economies - Brazil, Russia, India, China and South Africa - issued a statement calling for a continued commitment to open worldwide trade and support of a multilateral trading system via a strengthening of the World Trade Organization (WTO).
Economic troubles could spread much further if Trump goes through on a threat to impose auto tariffs on trading partners including Europe and Japan, the International Monetary Fund warned in a pre-G20 report.
Two days after GM's announcement, the U.S. Trade Representative Robert Lighthizer released a statement saying he would "examine all available tools to equalize the tariffs applied to automobiles" between the U.S. and China at the president's direction.
While Trump's meeting with Xi is all but certain to go ahead, the USA president on Thursday abruptly scrapped his planned talks with Putin, citing Russia's recent seizure of Ukrainian vessels.
Although the new pact inherits key features from the old one, Trump has declared it a victory for the USA workers he claims were cheated by Nafta and yesterday called it the most "modern and balanced agreement in history".
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The longest living president in USA history, one of his biggest achievements was steering America through the end of the Cold War. He also battled Parkinson's disease and was hospitalized in the past year for a blood infection, low blood pressure and fatigue.
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Grande also reenacts scenes from " 13 Going on 30 ", " Bring It On " and " Legally Blonde ". A trailer for a music video (which are usually only three minutes long).
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The migrants endured hellish conditions upon arriving at the Mexican border town of Tijuana after travelling up to 4,500 miles. A lot of them remain determined to enter the US , although some have accepted temporary work and asylum from Mexico .
On climate change, the American president holds different views from numerous other G20 leaders.
Trump met Friday with Japanese Prime Minister Shinzo Abe and Indian Prime Minister Narendra Modi in a rare trilateral meeting.
The Kremlin said it had not been notified and only learned about it from the tweet.
Russian President Vladimir Putin and Saudi Crown Prince Mohammad Bin Salman high-fived each other at the beginning of the G20 forum.
US President Donald Trump's trade war with China is a dominant theme at the weekend summit but one sideshow is how leaders deal with the presence of Saudi Crown Prince Mohammed bin Salman. He had just finished lauding the "robust" relationship between the US and Japan, particularly in their trade cooperation.
In remarks to the summit relayed by the Xinhua news agency, Xi reaffirmed his pledges of economic reform "with increased efforts in intellectual property rights protection and more imports".
French President Emmanuel Macron said he told the prince in a separate meeting that Europeans will insist on global experts being part of the investigation into Khashoggi's killing, Reuters reported.
Saudi Arabia has said the prince had no prior knowledge of the murder.
Trump said there were "good signs" for a potential trade deal with Beijing ahead of a Saturday dinner meeting with his Chinese counterpart Xi.
He said Trump "gets along" with Putin, as he does with the leaders of a number of the G20 countries. However, it is being reported that Ms Merkel still plans to have her one-on-one with Mr Putin, with the pair set to meet on Saturday morning.