The last Soviet leader, Mikhail Gorbachev, on Thursday urged Donald Trump and Vladimir Putin to restore trust between the United States and Russia a day ahead of their first meeting in Hamburg.
"Above all it is good that this meeting is finally happening, but a shame that it is only coming now," Gorbachev told Russian state news agency RIA Novosti.
"A lot of time has been lost that they need to make up for. They need to rebuild trust."
Ties between Moscow and Washington are at their lowest point since Gorbachev helped drag them out of the deep freeze in the 1980s.
The US has slapped sanctions on Russia for its meddling in Ukraine and the two countries are on opposing sides over the war in Syria.
Trump repeatedly pledged to improve ties with Moscow during his presidential campaign and lavished praise on Putin.
But expectations of any breakthrough at the sit-down on the sidelines of the G20 summit are low as accusations Putin ordered a hacking and influence campaign to get Trump elected have made any concessions to Moscow toxic for the White House.
Gorbachev compared the meeting to his first encounter with US counterpart Ronald Reagan in Geneva in 1985, insisting some members of Reagan's administration tried to stop him attending.
"But he didn't give into pressure and for our part we came to meeting with serious constructive suggestions," Gorbachev said.
"Now the leaders need to give things a push...They need to put everything on the negotiating table and establish a mechanism for cooperation."