"I'm told if he doesn't get to the Finals, he's leaving. I don't know if that's true or not," Smith said, per Bleacher Report's Howard Beck.
Antetokounmpo is in the third season of a four-year, $100 million contract with the Bucks.
Smith's comments are in line with a May 2019 report from ESPN colleague Malika Andrews.
"In more concrete terms, a source close to Antetokounmpo said that getting to the NBA Finals is not just an ambition, it could tip the scales as he weighs his contractual future," Andrews wrote. "And if they can reach the NBA Finals next season, the Bucks can improve their chances of signing Antetokounmpo to the supermax in the summer of 2020."
In September, the Bucks were fined $50,000 by the NBA after general manager Jon Horst said publicly the team was "of course" planning to offer the four-time All-Star a supermax extension once eligible.
Antetokounmpo has shied away from discussing his contract situation, stating before the start of the 2019-20 season it wouldn't be fair to his Milwaukee teammates.
"I feel like if you have a great team, and our goal is to win a championship and be the last team standing and get better each day, I think it's disrespectful towards my teammates talking about my free agency and what I'm going through," he told reporters. "So when the time is right, we're all gonna talk about it. I don't think the time is right."
Meanwhile, the Greek Freak has continued to dominate by averaging 29.6 points, 13.8 rebounds and 5.8 assists to move himself to the forefront of the MVP race once again.
The Bucks own the NBA's best record at 53-9, six wins ahead of the Los Angeles Lakers, and lead the shortlist of legitimate championship contenders heading into the regular season's final quarter.
Antetokounmpo becomes eligible for the five-year supermax extension July 1.