The one attribute the Celtics never lacked, even in their darkest days this season, is confidence. That will serve them well now that their basketball backs are against the wall headed back to Milwaukee for a must-win Friday night.

But it’s also why they find themselves cornered, facing the conclusion of their season in the first place.

The overconfident Celtics engaged cruise control in the worst possible fashion at the worst possible time Wednesday night. They thought they had Game 5 of the Eastern Conference semifinals in the bag. But that bag had a viper waiting inside in the defending NBA champion Milwaukee Bucks, and when the Celtics fooled around it jumped out and sunk its fangs into them.

What a waste — wasted effort, wasted lead, wasted opportunity, and hopefully not a season laid to waste. That sums it up for the Celtics after their fourth-quarter fall from grace. A 14-point lead and certain control of the series evaporated in astonishing and disheartening fashion in a 110-107 loss to the resourceful and resilient Bucks.

Now, the Celtics have to do what neither team has done in this series, win two straight, or the last three-plus months of basketball brilliance gets zeroed out.

The undermanned Bucks, minus Celtics killer Khris Middleton, have certainly displayed the heart of a champion if not the shooting touch of one in this series. But if Milwaukee prevails, the Celtics will have no one to blame but themselves. Opportunity surrendered to the septic system.

“We’re hot just because we know we did it to ourselves,” said guard Marcus Smart. “Nobody else. We take the blame. But now we turn around and go right back at it. We ain’t got time to sulk. We ain’t got time to put our heads down.”

One game after they showed their growth by clawing back with a fourth quarter for the ages to tie the series, the Celtics displayed their growing pains and inability to handle good fortune with the requisite maturity in gift-wrapping this game. The path to the NBA Finals is visible in the East, but the Celtics tripped over their inflated egos. It could be a fatal misstep.

The Celtics often do their best work when the self-generated degree of difficulty is high. Mission accomplished on that front.

In the fateful final frame, the Celtics abandoned the ball movement, balance, and blinders-on focus that had gotten them that lead and seeming control of the series. Instead of staying the course, they went for the Waze shortcut to victory, reverting to hunting mismatches and knockout punch shots to take out the Greek Freak and Friends.

“We moved the ball well throughout the game, but that last 6, 6½ minutes we became stagnant and made it tough on ourselves,” said Jayson Tatum, who scored 12 of the team’s 21 fourth-quarter points on his way to a team-high 34.

Boxing allusions are as time-worn and hoary in sports writing as dust-covered typewriters, but they ring true. The problem with rearing back for the knockout blow is that you leave yourself vulnerable to counters — in this case, offensive rebounds, 17 of them, including seven by the Bucks in the fourth.

Milwaukee took full advantage, defying the odds and the Boston-skewed box score to squeak out the win on Bobby Portis’s put-back of a Giannis Antetokounmpo missed free throw with 11.4 seconds left, putting the Bucks up 108-107. It was the last of Milwaukee’s 20 second-chance points. Big rebounds and timely threes by the Bucks stung the Green.

Adding insult to incredulity in this collapse, Bucks guard Jrue Holiday, who tied the game at 105 with a clutch trey, out-Smart-ed Defensive Player of the Year Smart.

On Boston’s penultimate possession, Holiday blocked Smart’s potential go-ahead hoop and then threw the ball off Smart to gain possession. Then on the final Boston possession, Holiday relieved Smart of the ball.

The Celtics’ core trio, Tatum, Jaylen Brown, and Smart, all of whom had their magic moments while combining for 75 points, came out to perform the parquet postmortem, to examine how they had the reins of this series in their hands and then just let them slip right through.

Tatum provided the most positive spin and mien. Smart and Brown, who saw his 26-point, 8-rebound, 6-assist night go for naught, were smoldering.

But how do you justify the unjustifiable, explain the unexplainable, and answer for the unacceptable? What valid reason could possibly be provided for blowing a game you led by 14 with 10 minutes to go?

How do you squander a contest in which you shot 51.2 percent to Milwaukee’s 43.5 percent, in which you outscored the Bucks, 50-44, in the paint despite Antetokounmpo going for 40, in which role guys Derrick White and Daniel Theis shot a combined 9 of 10 for 20 points, in which Milwaukee had 13 turnovers to your 10?

“We gave it away,” said Brown, who caught fire for 16 points in the third but went scoreless in the fourth after coach Ime Udoka iced him on the bench. “That’s how we lost it.

“Give credit to Milwaukee. They played hard for 48 minutes. We played hard, but we weren’t as purposeful down the stretch as we needed to be, and it cost us … cost us the game.”

Smart said the Bucks “out-prided us.”

Actually, the Celtics didn’t lose because of a lack of pride, but too much of it. Hoops Hubris, their trip-wire, caught up to them and caught them.

It’s hard to survive in the playoffs when you give away games you’re supposed to win, but as Brown said, “if anybody can do it the Celtics can.”

“The series is not over,” said Tatum.

The Celtics better hope the Jays are offering more than forced positivity, and that they can look back on this as a difficult lesson learned, not a self-destruct mechanism that blew up their championship chase.

Christopher L. Gasper is a Globe columnist. He can be reached at christopher.gasper@globe.com. Follow him on Twitter @cgasper.

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