Written in Portuguese on one wall above center court at Carioca Arena is the phrase “Um mundo novo.” Translation: A new world.

It didn’t seem that way in men’s Olympic basketball — until now.

Threatened by Australia in its previous game, the U.S. men’s team survived a heart-racing final seconds to defeat disciplined and experienced Serbia 94-91 on Friday night and extend its winning streak in international tournaments to 49 games.

No. 50 is no given, and suddenly, a gold medal that seemed a formality just a few days ago is anything but certain.

The Americans, boasting a roster stuffed with NBA All-Stars, elite outside shooters but only two former Olympians, look very vulnerable and somewhat lost on the world’s hardwood stage.

“We do have more talent,” said U.S. coach Mike Krzyzewski, who has been warning about overconfidence since his team’s arrival in Brazil. “We have to get our talent playing even much better as a team.”

Despite a ferocious start, the Americans couldn’t put away the underdog Serbs, who ran their cut-and-cut-again offense with precision and had a chance to tie it in the final seconds. But guard Bogdan Bogdanovic’s 3-pointer from the left wing was a little long and Kevin Durant secured the rebound and the relieved Americans walked off the floor with their streak intact but their standing shaken.

Kyrie Irving scored 15 points and Durant and Carmelo Anthony added 12 apiece for the U.S. team, which arrived in South American lauded as the latest version of an American super squad. However, after their 10-point win over the Aussies earlier this week and now a nail-biter against the Serbs, the rest of the field might be starting to believe the U.S. can be taken.

“We got in our own heads,” Durant said. “We had a great

start, great start. We were up almost 20 points. We should have held the lead and we just got sidetracked by stupid stuff, from the calls to the physicality, the extra plays. We got to stay with it. That’s the way it’s going to be out here.”

Nikola Jokic, who plays for the Denver Nuggets, scored 25 and Milos Teodosic and Miroslav Raduljica 18 each for the Serbs, who fell to 1-3 but left the building feeling confident after making the Americans sweat.

Paul George said Serbia’s systematic offense was unlike anything he and his teammates have faced.

“Once again, we relied on natural talent,” George said. “This is why these guys are special in our league. These international guys really know how to move and really know how to cut. It’s more about how they’re running their offense. It’s wearing us down.

“It’s like they don’t get tired.”

It was the first Olympic game between the nations, and a rematch of the 2014 Basketball World Cup championship won by the U.S. 129-92. However, this never resembled that track meet as the Serbs slowed the tempo and forced the Americans into turnovers and rushed possessions. The world’s best team certainly didn’t look or play like it for long stretches.

The U.S., which will wrap up preliminary-round play against France on Sunday, led 94-87 with 2:11 left on a basket by Anthony. But the Serbs got a basket by Jokic and two free throws Teodosic to get within three.

Durant, who only attempted four shots, misfired with eight seconds left giving Serbia one last chance. After a timeout, the ball wound up with Bogdanovic, who had a great look from 22 feet but was just off the mark.

“We are that kind of a team that we never give up and we showed this today,” said Teodosic, one of Europe’s craftiest guards for the past decade. “This is third game in a row that we have a very bad beginning, and especially (against) the teams like the United States, it’s very tough to get back in the game when you are losing by 10 or 15 in the beginning.”

The 10-point win over Australia — just the fifth time the U.S. has won by 10 or fewer since 2006 — raised the possibility that this American team might not be as golden as previous ones.