The Crimson also arrived unbeaten in their first seven league games and with a glittering 21-2 record, which had boosted coach Tommy Amaker’s team back into the Top 25.

Not anymore.

Harvard (21-3, 7-1) left with yet another loss at Princeton, as a second-half surge by the Tigers, in which they made 62.5 of their shots (15 for 24), put the Crimson away, 70-62, leading to fans streaming onto the court in celebration of Princeton’s 23d consecutive home win over the Crimson.

“A tough loss for us,’’ said Amaker. “I told our kids that Princeton deserved to win. I thought they played an exceptional second half and did an outstanding job of getting the ball into the post. We didn’t shoot well. I certainly thought they deserved to win.’’

The Tigers (13-10, 4-3) did earn the win, off a first half after which they trailed by 5 (27-22). The Tigers focused on what many perceive to be Harvard’s main flaw - a lack of size. Forward Ian Hummer (20 points, nine rebounds) and 6-foot-11-inch center Brendan Connolly (11 points, six rebounds) were especially effective down the stretch.

Princeton fought its way back in the second half, building a 52-47 lead with 3:57 left. Amaker called a timeout to settle down his team, and Brandyn Curry responded with a floater to make it 52-49. But Princeton looked like a team that knew how to win a game like this - at least against Harvard.

A jump shot by Hummer restored the Tigers’ lead to 5 at 54-49 with 3:11 left.

One minute, 11 seconds later, the lead was 10 and the game seemed over. Harvard cut its deficit to 6, but it wasn’t enough.

“They wanted it more than we did,’’ said forward Keith Wright, who led the Crimson with 16 points and 12 rebounds. “Most of the time the team that wants it more comes out with the victory. We didn’t step up. They’re a good team, they play hard every time we play them. I think I’ve beaten them one time since I got here. It’s a tough place to play when they are at home. We wanted to beat them here, it’s something we talked about. Unfortunately, we didn’t get it done tonight.’’

With every other Ivy team having at least two league losses, a win against Princeton would have solidified Harvard’s grip on first. With six games remaining - four at Lavietes Pavilion, where they haven’t lost in two years - it would have taken a major meltdown for the Crimson not to claim the outright title.

If Amaker had to do anything last night, it was to quietly remind his team that the good feelings coming off a tough win against Penn Friday night would evaporate in an instant with a loss to Princeton. And if he needed an extra motivational tool, all Amaker had to do was mention the last time the teams met - last year’s Ivy League playoff game, when a Douglas Davis last-second jump shot beat the Crimson and cost them an NCAA Tournament bid.

Last night, the Crimson led for a while, but it didn’t last. “We just couldn’t get over the hump and get anything going,’’ said Amaker.

The Tigers have won 26 Ivy League titles and have made 24 NCAA Tournament appearances, but with three league losses, Princeton’s chances of getting another Ivy crown were remote, at least before the game.

Princeton certainly didn’t play like an also-ran at the start, running its offense smoothly and controlling the tempo for much of the first half. Although the Crimson squeezed out a halftime lead, largely because of the shooting of freshman guard Corbin Miller (8 points) and the work underneath of Wright, who had 10 points and six rebounds in the first 20 minutes, Princeton’s patience and precision made it clear it would not go away.

From the 14:25 mark of the second half, when the game was tied at 36, until the final minutes, the biggest lead the Crimson could manage was 4 points, and when the Tigers finally got the lead with 7:28 left, the climb became that much steeper for Harvard.

Now the Crimson must rebound from their first Ivy league loss.

“We wanted to go unbeaten,’’ said Wright. “Obviously, that’s not the case now. We just have to move on and get past it and focus on the next game.’’

Mark Blaudschun can be reached at blaudschun@globe.com.