Putin and Trump are to hold their first meeting there.
"I think what they're hoping to pull off is a world without the USA", said Dennison.
'It's not only going to be about (economic) growth but rather sustainable growth, ' she said.
Referencing Trump, she said: 'We know certain positions that the American government holds and I don't expect that these positions will be abandoned as a result of a two-day trip to Hamburg'.
From Tuesday to Saturday, Xi is to pay a state visit to Germany, and then attend the annual G20 summit in Hamburg.
Xi and Putin also met on Monday where they agreed to further cement bilateral ties and strengthen coordination on the Korean Peninsula and other major issues.
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Trump announced on Twitter that he would be talking on the phone with leaders from Germany, Italy and France on Monday. Last week, the US approved $1.42 billion in arms sales to Taiwan.
About 21,000 police officers from across Germany have been sent to protect the meeting of the representatives of the world's 20 largest economies, Euronews.com reported.
Earlier in the weekend, Mr. Trump had called Chinese President Xi Jinping, and "reiterated his determination to seek more balanced trade relations with America's trading partners", the White House said.
The gathering outside city hall took place in parallel with protests by canoeists on the nearby river Alster, while in the port of Hamburg, Greenpeace staged a climate demonstration near a ship laden with coal.
Eberhard Sandschneider, a China expert at Berlin's Free University, said Merkel's main goal was to gather as many allies as possible on climate, trade and Africa before the G20 summit, describing the encounter as a pure "coordination meeting".