Former Scotland manager Alex McLeish insists the national side face a tough test in former player Sergej Milinkovic-Savic and Serbia.
But he believes Steve Clarke 's men can get the job done in Belgrade to reach a first tournament in 22 years.
Clarke replaced McLeish as manager in May 2019 but the journey to Thursday's playoff began under the former Rangers boss when he capped a successful Nations League group with a 3-2 win over Israel in November 2018.
He's been impressed with the defensive resolve of the Scots, in particular Manchester United star Scott McTominay 's recent performances as a centre-back - though he warned the country of Lazio playmaker Milinkovic-Savic, who played under McLeish in Belgium at Genk.
Speaking to Sky Sports, McLeish said: "We've all got to be optimistic and say, 'it's now or never'. Although it's not now or never, because there have been great signs over the past few weeks.
"But it's a tough team and a tough place to go to. They've got guys like Aleksandar Mitrovic and Sergej Milinkovic-Savic, a young guy I had at Genk.
"He's in the form of his life as well, so they're very dangerous players."
Asked how he rated the national side's last outing, a 1-0 win against the Czech Republic in the Nations League, McLeish answered: "Brilliant.
"The Czech Republic game, away from home, you're saying well, we should be beating them because it was a third-tier team. Well, we beat them here and it wasn't a third-tier team.
"It was a great result, we had moments where it was a bit scary but we were very organised and big McTominay going back into the centre-back position has been a great move for us as well.
"It shows you he's versatile, he can move into midfield and add a new dimension to his game."
