A Yanukovich victory could see the country of 46 million people shift back towards former Soviet master Russia after five years of infighting and a sliding economy turned the euphoria of the Orange revolution into frustration and disappointment.
Both candidates pledged integration with Europe while improving ties with Moscow, but Tymoshenko is seen as more pro-Western. Yanukovich is unlikely to pursue membership of NATO, an 'Orange' goal that infuriated neighbouring Russia.
Yanukovich, 59, a beefy ex-mechanic, called on Tymoshenko, 49, to resign as prime minister. But Tymoshenko's team said in the early hours of Monday they had counted 85 percent of votes and she was leading by 0.8 of a percentage point.
Each side accused the other of fraud, but Tymoshenko stopped short of repeating a threat she made last week to call people out onto the streets if she believed the election was unfair.
If the voting pattern remains unchanged, Yanukovich would be the first president since independence in 1991 not to receive more than 50 percent of the vote, although elections in the 1990s were not always judged free and fair.
Some analysts said that given the relative weakness of his position he might seek a compromise with Tymoshenko. They did not rule out the possibility that the two camps were talking.
Under this scenario, Tymoshenko might concede defeat in exchange for remaining prime minister and living to fight for the presidency another day. Tymoshenko was the co-architect of the 2004 revolution with pro-Western President Viktor Yushchenko, but their relationship quickly soured.
Looking stern in front of reporters, the fiery former gas tycoon late on Sunday urged her team to "fight for every result, every document, every vote." The tone was moderate and analysts said they doubted Tymoshenko could stage a repeat of 2004.
She was scheduled to hold a news conference later on Monday.
A disputed outcome would further delay Ukraine's chances of repaying more than $100 billion (64.2 billion pounds) of foreign debt and nursing its economy back to health after a 15 percent collapse last year.