The Knicks have looked like a league-average team this season, each hint of momentum followed by a slam of the brakes. 

So it tracked that an afternoon that began with a literal high point ended with another low. 

The Knicks set a franchise record for points in a quarter in the first, yet proceeded to surrender their most points in a game in two-plus seasons under Tom Thibodeau. An electric start gave way to a defensive collapse in a 145-135 loss to the Thunder in front of 19,812 at a sold-out Garden on Sunday afternoon. 

The Knicks (6-7) slipped back under .500 with another loss that came on the heels of a victory. They have alternated wins and losses for seven straight games, generally beating teams they are supposed to beat and losing to teams they are supposed to lose to. 

The Thunder (6-7) are in neither category, but they feature an emerging star the Knicks lack. 

After a slow start, Shai Gilgeous-Alexander — a potential trade target, should the middling Thunder decide to tank for a lottery pick — sliced through the Knicks’ defense at will for 37 points on 13-of-22 shooting with eight assists, three steals and two blocks. 

SGA far outdueled RJ Barrett, looking like the more promising two-way wing while generating whatever shot he wanted — often against Barrett, who was a nonfactor and benched after picking up his fourth foul early in the third quarter. 

“There was no resistance,” Thibodeau said after the Knicks surrendered their most points since Dec. 27, 2006 — when an Isiah Thomas-coached team also allowed 145 in a triple-overtime win over the Pistons. 

They had not allowed as many as 145 in regulation since a 145-129 loss to the Kansas City Kings on Dec. 8, 1979. The Knicks have not given up more points in a game since March 24, 1979, when they lost 148-143 to the Bulls in double-overtime. 

Even amid OKC’s offensive deluge, a strong offensive start — including a franchise-best 48 points in the first quarter — had the Knicks within 96-93 at 6:08 of the third quarter before a backbreaking Oklahoma City run put the game on ice. 

The Thunder poured in 26 points in the final six minutes of the quarter, running their offense through Gilgeous-Alexander. In the third quarter alone, Gilgeous-Alexander went 7-for-10 with just one attempt (a swish) from beyond the arc. Whomever the Knicks threw at the 24-year-old with Barrett on the bench, from Cam Reddish to Derrick Rose, the long, lightning-quick Gilgeous-Alexander put on his hip. 

“They were ready to play,” said Jalen Brunson, who along with Barrett was benched late in the game. “Made a couple more plays than us, and we should be walking out of here with a win, scoring that many points. 

“Just defensively, we were just — myself included — we just weren’t right and unacceptable.” 

The Knicks, who led by 13 in the first quarter, still managed to hear boos two periods later. They trailed by as many as 19 in the fourth, in which a strong bench effort — including 24 points from Immanuel Quickley — helped them claw back, but they only got as close as nine points. 

Gilgeous-Alexander was so strong he overshadowed his own teammate’s triple-double, as Josh Giddey finished with 24 points, 12 assists and 10 rebounds, while Lu Dort added 24 points. The Knicks allowed the Thunder to shoot 17-of-31 from beyond the arc. 

“Can’t just let people get what they want,” said Barrett, who went 2-of-10 for four points. “Can’t let one guy have 30-something, another guy have 20-something, another guy gets a triple-double. 

“We can’t allow stuff like that. It was too easy for them.” 

It was too easy for the Knicks in the first quarter, riding a balanced attack — nine different Knicks scored in the period, and Jericho Sims and Obi Toppin seemed to be alternating possessions of finishing alley-oops — while they shot 64.5 percent and turned the ball over once. 

But as has been the story of the season, the good feelings went sour quickly. The Thunder combined for 86 points in the following two quarters. 

“We have to take a hard look at [the defense] and then evaluate everything,” Thibodeau said. “And what’s the best way for us to have a chance to be successful.” 

It will only get tougher for the Knicks, who embark on a five-games-in-seven-days trip that begins Tuesday in Utah and swings Wednesday to Denver for a back-to-back that Evan Fournier called “probably the toughest in the league” because of the altitude and conditions in traveling. 

“When you go west, it’s always a tough time, regardless of your dynamic,” said Fournier, who went 0-for-5 and was held scoreless. “It’s about how you respond.” 

This season, the Knicks have not.