If confirmed, the result would mark the return of the West-leaning Tymoshenko and Yushchenko, who led the "Orange Revolution" in 2004 that swept the pro-Russian Yanukovych from power.
Tymoshenko, who said she hoped to form a new government with Yushchenko within 48 hours, now looks set to oust Yanukovych as prime minister.
Al Jazeera's Jonah Hull reported that Tymoshenko will hold coaltion talks on Monday with Yushchenko.
This would return her to a post she held in the aftermath of the Orange Revolution before falling out with Yushchenko.
"The Orange Revoltion has been saved by Tymoshenko's election results. She saved it from oblivion," Taras Kuzio, a Ukraine specialist at George Washington university, said.
Ukraine, which has held three national polls in as many years and suffered months of constitutional paralysis, is notorious for the complexity and rancour of political deal-making.
Some analysts question whether Tymoshenko will be able to overcome previous personality clashes with her partner Yushchenko.
They also point out that Yanukovych, whose Regions party is forecast to have won more than a third of the vote, remains a major force.
It was unclear whether Yanukovych would dispute the results, but the Regions party announced at a rally on Kiev's main square that court challenges were a possibility.
"We have to wait for the reaction of the Regions party.... They will go to court and they will try to mobilise protests against the election," Nico Lange, an analyst at the Konrad Adenauer Foundation in Kiev, said.
Russia had strongly backed Yanukovych and saw the pro-western Orange Revolution as a crushing foreign policy defeat - straining relations with both Yushchenko and Tymoshenko.
In Moscow's first reaction, Viktor Chernomyrdin, the Russian ambassador, said late on Sunday, that "we will work with any government".
But Russian parliamentary deputies and experts cast doubt on the viability of an Orange victory.
"Regions party won the moral triumph," Konstantin Kosachev, head of the parliament's foreign affairs committee, told Interfax.