In 1997, Kazlauskas became head coach of the Lithuanian national basketball team, and invited Sireika to join the squad as his assistant.

In the 2005–06 season Sireika was dealing with huge amount of criticism aimed at him, because of poor results in EuroLeague and Lietuvos rytas Vilnius was at the top of the Lithuanian league standings.

On March 28, 2006 in a LKL regular season rivalry game, Sireika's coached Žalgiris played against Lietuvos rytas and lost the match by 30 points.

In EuroBasket 2005, Lithuania was named as one of the contenders to win gold and defend their 2003 title, Lithuanian team finished first in the group and entered quarter finals, where they lost 47–63 to France led by Tony Parker.

The 2007–08 season did not start out well, as Russia's basketball experts had expected[citation needed], UNICS was 8th in the Russian Super League, Sireika was dismissed from the head coaching position and moved to assistant.