Even as the nation faces conflicts on its eastern and southern borders, the youthful entertainer's resounding victory at the polls comes as a brutal blow to incumbent Petro Poroshenko, a life-long politician with deep ties to the country's old-guard bureaucracy.
"Much remains to be accomplished to fully realise the peaceful, democratic and prosperous Ukraine that its citizens have called for", Tusk and Juncker wrote.
"You can count on the EU's strong support to Ukraine's reform path", both European Union leaders said, which included goals such as consolidating the rule of law, fighting corruption and economic stability.
The comedian and actor, who played his country's president on television, is now set to take on the job in real life after winning Ukraine's presidential runoff vote in a landslide, according to an exit poll released Monday with almost all the votes counted.
"It is very important to be guided by reason during the vote, because it's not amusing", he said.
Critics accuse the president-elect of having an unhealthily close working relationship with powerful oligarch Ihor Kolomoisky, whose TV channel broadcasts his comedy shows. However, the diplomat said he felt some "cautious optimism".
For many of his supporters, Zelensky, 41, represents a new political generation.
Zelenskiy rode a wave of support in Ukraine's west and east - areas that have been traditionally polarized - on his path to the presidency. "I have no doubt that Ukraine's new leader will stick to the rhetoric toward Russian Federation he used during the campaign".
The US also could not help but congratulate ordinary Ukrainians for holding elections, which, according to Washington, "reflect Ukraine's vibrant democracy" that managed to survive despite what the State Department called "years of unrelenting Russian aggression".
Anatolii Stepanov AFP Getty Images
For now though, Zelensky, despite years in the public eye as a performer, remains an unknown quantity.
"We need to see what the first decisions are, the first appointments".
Poroshenko's unpopularity stems from unhappiness over the economy, over corruption, and over the war with Russian Federation in Eastern Ukraine.
Officials in Moscow on Monday sounded cautiously optimistic - but also sounded a note of condescension regarding Zelenskiy's victory.
But its fate hangs in the balance after a Kiev court ruled days before the election that the change of PrivatBank's ownership was illegal, delighting Kolomoisky but rocking the central bank which said it would appeal.
Zelenskiy has pledged to keep Ukraine on a pro-Western course, but has sounded less emphatic than Poroshenko about possible plans for the country of 42 million people to one day join the European Union and North Atlantic Treaty Organisation.
NATO Secretary-General Jens Stoltenberg tweeted his congratulations, adding cautiously: "Ukraine is a valued NATO partner and we look forward to continuing our cooperation".
Tusk and Juncker said further implementing Kiev's trade and political pact with the European Union could provide "crucial" help.
If borne out by election returns, the overwhelming victory by Zelenskiy would stand as a crushing rebuke to Poroshenko's five years in office and a verdict on the country's deep-seated corruption, sickly economy and grinding war against Russian-backed separatists in the country's east that has killed over 13,000 people.
Zidane: Benzema Is The Best No.9 At The Moment
The striker scored all three goals as Madrid claimed a flattering victory and Marcelo paid tribute to his team-mate. We have to find the motivation. "We also started well, with rhythm, and just lacked a goal in the first half".
Iran feels the heat as USA scraps oil sanction waivers
He added the length of the sanctions being at zero depends on the Iranian Ayatollah and "his cronies" meeting certain demands. Iraq had obtained a three month waiver last March for gas and oil imports from Iran.
Bruins defeat Maple Leafs, force series back to Boston for Game 7
Who knew it would come on shorthanded? Kapanen was also credited with an assist on Matthews' goal, along with Muzzin. If they win, they'll advance to the second round for the first time since 2004 and face the Columbus Blue Jackets.