The Tuscans are currently winning a four-team battle to avoid the drop with Palermo and Genoa two points behind and troubled Pescara a further seven points from safety.
But 16th-placed Chievo are only six points clear of Sunday's opponents and, with the Gialloblu taking just one point from their last three games to Siena's five, Hetemaj knows defeat at the Stadio Artemio Franchi could have dire consequences.
The Finn told the Corriere dello Sport: "It's clear Siena will be giving their all to bring the three points home, but we want to win too.
"It's an extremely important game and the five games that follow will be just as important.
"Every Sunday offers the opportunity to take points home and we still need a few more before we can relax.
"The teams behind us have started to catch up so we have to do everything in our power to keep on top of them."
Defender Nikos Spyropoulos is the only injury concern for Chievo, who have failed to beat Siena in five attempts.
The Bianconeri, meanwhile, must cope without injured defender Carlos Matheu.
Head coach Giuseppe Iachini reflected on the progress he has made since replacing Serse Cosmi in December.
"We were nine points off Pescara and had to scramble our way up," he said.
"Slowly we brought players into the team who had certain characteristics and we started getting results without spending much money.
"We are happy just to be in a position where we can fight for our survival."
Tomorrow's match represents Siena's best opportunity to take all three points over the season run-in, with subsequent matches against European hopefuls Roma, Catania, Fiorentina, Napoli and AC Milan promising a testing conclusion to a difficult campaign.