After pro-Russian Yanukovych was accused of vote-rigging in the 2004 presidential election, massive "Orange Revolution" protests helped pave the way to pro-Western Yushchenko's election victory.
But the two men now share power after Yanukovych won this year's parliamentary election and was named prime minister with broad powers.
"Regarding the president's decrees, we do not know how they are prepared. We do not expect them and we do not know what is the aim of them," Yanukovych said Monday. "A number of decrees are adopted unpredictably."
Yanukovych and Yushchenko have frequently clashed over foreign policy, Cabinet appointments and other government business.
Under constitutional reforms that took effect this year, Yushchenko cannot fire Yanukovych and Yanukovych cannot ignore the president.
The law provides little guidance over how power should be divided.
In many ways, Yanukovych already holds the upper hand. As head of government, he controls the country's finances.
His party also has allied with other political groups, effectively giving it a majority in parliament.
On Monday Yanukovych said that defining the functions of the cabinet and of the president was "very difficult and very painful."
Yanukovych also said that representatives of Yushchenko's office had failed to take part in drafting next year's budget despite an earlier agreement.
Last week, the government and the president struggled over the budget as the president vetoed it and the governing coalition failed to override the veto.
Yushchenko said last Thursday that his relations with the premier were worsening, noting it was not his fault.
A day later the two men met and pledged to solve their differences.
Source: International Herald Tribune