Jason Terry and the Boston Celtics forced a Game 5 in their series with the New York Knicks on Sunday. Getty Images

Yeah, no, nope, naw, not today. That's what the Boston Celtics said Sunday. This is not to say they are going to win this first-round playoff series against the New York Knicks. They will almost surely not.

They remain down three games to one, staring up at a rough cliff, and only the daffiest optimist would give them a chance. Let's be real. The second-seeded Knicks are quicker and stronger and the seventh-seeded Celtics are erratic and…erratic. For Boston, this is probably not going to end well.

But it did not end in Game 4. It was supposed to be the end, wasn't it? The script had been pre-written. End of an another Boston era. Farewell to Kevin Garnett, who came here and delivered. Maybe even to Paul Pierce, who'd been in green forever, in sickness and in health, the throaty-voiced heart and guts of the whole revival. These 21st-century Celtics had won a title, and come maddeningly close to another, and there was that near-shocker against the Miami Heat last season, but there were finally too many miles on the rumbly old Volvo. The Celtics appeared creaky and slow. They faded when it mattered. When Rajon Rondo went down with a season-ending knee injury, there was a flurry of winning, and a brief delusion that Boston was better off without its electric point guard, but it was a mirage. Three games down was three games down. Since the Marathon attacks, Boston had been such an unbreakable, stirring place, but a city's emotion couldn't transform reality. Time had caught the Celtics.

Paul Pierce said he didn't think about it. He didn't think about the big picture, the idea of losing Game 4 and what it meant for him and his career and this town, the only pro city he has ever played for. He didn't sound ready to go there. Contemplation? He wasn't contemplating a thing. Actually, that's not true. Pierce said he called friends in New York before the game and told them to be ready for dinner this week. Dinner in New York. Game 4 was going to be Game 4. Pierce had penciled in Game 5.

Paul Pierce and the Boston Celtics are going back to New York after their Game 4 win Sunday. European Pressphoto Agency

"I was pretty calm today, actually," Pierce said. "I really didn't think about the aftereffects."

Still, the afternoon began desperate. That's what endings are like for declining champions: cornered, angry, furious. There wasn't much of a crowd when the game began at TD Garden, owing partly to the glorious New England spring weather outside, but also to…well, who wanted to witness the brutal finale of a beloved team? But the Celtics opened with energy, and New York was awful. The first half was a rout. Early in the third quarter Boston led by 20. The Knicks seemed ready for a late brunch.

But the Celtics would make it hard on themselves. Of course. They would allow the Knicks to claw back into it. Of course. Inconsistency has been the consistency of these Celtics, and soon they were letting the Knicks in through the kitchen door and fixing them pancakes. An easy day unraveled. New York charged back in the third quarter and took its first lead late the fourth. Knicks fans began dancing courtside in blue and orange. Even when the game kicked into overtime, it appeared to be New York's game to lose.

And then a strange thing happened: New York lost. Or, rather, the Celtics did exactly what no one expected, which was to find improbable energy and take the game away. The veteran guard Jason Terry scored Boston's last nine points in a 97-90 overtime win. Terry had been fitful all season; before Sunday, his biggest moment in this series had been the crisp elbow he took from New York guard J.R. Smith, which earned Smith a one-game suspension. But this Terry? This was the Terry who'd tormented the Heat in the 2011 Finals while playing for Dallas.

Now Terry was standing outside the Celtics locker room talking about the next game in New York. Boston planned to be ready. "I don't care if Patrick Ewing comes out and suits up for the Knicks," Terry said.

"We have nothing to lose," said Boston's Jeff Green. "All the pressure's on them."

Ridiculous? Sure. Here's an even more ridiculous stat: Since 2004, no New York team carrying a 3-0 series lead over a Boston team has won another game. The Celtics cannot summon Curt Schilling in a bloody sock. But it will happily rely on some historical ghosts, even if it just messes with New York heads.

In the rafters of the Garden in Boston there's a grid-like series of banners with the retired numbers of Celtic legends. The youngest one contains the numbers of modern Celtics: Larry Bird's 33, Kevin McHale's 32, Robert Parish's 00. There are two empty slots at the bottom, side by side, where you could easily fit a 34 and a 5. This is not to say that Paul Pierce and Kevin Garnett have reached the end. You never know anymore when people are ready to walk away. But they weren't ready Sunday. For Boston, that felt like enough.

Write to Jason Gay at jason.gay@wsj.com