Too much fatigue and way too much Knicks defense.
Carmelo Anthony scored 23 points, Smith kept up his surge with 20 and New York snapped San Antonio's seven-game winning streak with a 100-83 victory on Thursday night.
Steve Novak added 15 points and Tyson Chandler had 10 points and 14 rebounds to help the Knicks bounce back from consecutive losses by dominating the final period against the Spurs, who appeared to run out of gas in their second game in two nights.
"They shot the hell out of it and I could see it wasn't going to happen," Spurs coach Gregg Popovich said. "Just too low on fuel and their defense was too good. Bad combination, and then they made shots, which makes it even worse."
Jackson played just three minutes off the bench before spraining his right ankle when he took a shot then fell back into a waitress working the sideline in front of Mayor Michael Bloomberg.
Smith, who had scored 25 points in his past four games as a reserve, highlighted his outing with an acrobatic dunk in the fourth quarter that brought fans to their feet. The lobbed pass came from reserve point guard Pablo Prigioni, who had one of his most complete games since coming to the NBA at age 35, finishing with six points and nine assists.
That meant the Knicks needed just an ordinary game from Anthony, who finished with about half of the season high-tying 45 he scored in a loss against Portland on Tuesday.
"That's their scheme, to try to take me out of the game," Anthony said. "Just a matter of us making shots when we swing the ball, and Novak made shots, J.R. kept his streak alive of playing good basketball, Pablo came in and led our team from the point guard position. Everybody contributed in their own way."
The Knicks put away what had been a close game for three quarters, scoring the first 10 points of the fourth to take a 17-point lead. The Spurs, playing for the fourth time in five nights, went with reserves from there and never put much of a dent in it.
San Antonio, which averaged 111.7 points on 53.4 percent shooting during its winning streak and rang up 117 points on Wednesday in Milwaukee, shot just 36 percent in its lowest-scoring game this season. Gary Neal led the Spurs with 12 points.
"It wasn't the first time, it's not going to be the last one," Spurs reserve Manu Ginobili said of the tired performance. "But of course we've got to give them credit because they forced us to not find anything easy, they made us really work."
Popovich praised the Knicks' defense before the game, though it had slipped during their stretch of three losses in four games. Looking for a fix, coach Mike Woodson gave Marcus Camby a start at forward in hopes of ending the Knicks' slow starts, and they allowed just 19 points in the first quarter.
"Our defense was slacking the last 10 games or so. Our defense kind of went the other way," Woodson said. "We put a nice feel-good tape together to show our players how we started the season, that we were No. 1 in defense and on offense. There was some major slippage."
Amar'e Stoudemire had 10 points on 4-of-10 shooting in his second game of the season after knee surgery. He seemed more aggressive than in his debut Tuesday but is clearly without his typical explosiveness, having a number of shots blocked when he tried to power his way inside.
Leading 67-60 after three, the Knicks made it 77-60 on Novak's 3-pointer with 10:24 remaining in the game. New York poured it on from there, the lead growing to 20 when Smith caught a lob from Prigioni around his waist while in the air and slammed it home as the players on the Knicks bench jumped up in celebration.
"J.R. does some amazing things. He's a freak of nature," Chandler said.
Novak followed with another 3-pointer on the next possession to make it 95-72.
Jackson entered the game just 13 seconds in, but his night was over late in the period after his bizarre injury. Jackson launched a 3-pointer from the corner with 3:23 left, stepping backward out of bounds after the miss and falling onto the waitress. He limped back onto the court as the Knicks played with a 5-on-4 advantage that led to Anthony's 3-pointer. The Spurs then took a timeout and Jackson limped off toward the locker rooms.
It was unclear if the waitress was serving Bloomberg, who was seen eating a box of popcorn shortly after.
Jackson was unavailable for comment.
Woodson said before the game he thought the Spurs were the best team in basketball right now, though Popovich dismissed that as gamesmanship he learned from Larry Brown. San Antonio sure didn't look its best while losing for the first time since a 112-106 setback at Denver on Dec. 18 and getting held well below its NBA-best 105.9 points per game.
The Spurs hadn't played here since January 2011, so this was their first time back in Madison Square Garden since renovations began. Popovich jokingly yelled at Parker for not knowing how to get from the locker room to the court before the game. ... Ginobili, Prigioni's teammate with Argentina's national team, was held to eight points on 2-of-10 shooting.
Kawhi, PG not entertained by Fitz and the Tantrums at Clippers' ceremony
The Clippers hold a ceremony breaking ground on new arena and the entertainment doesn't seem to entertain Kawhi Leonard and Paul George.
Will new arena help Clippers build their own identity?
Kendrick Perkins says new arena will help Clippers separate themselves from the Lakers and start fresh.
Damian Lillard pays homage to various iconic figures in new music video
NBA superstar Damian Lillard re-created some iconic images featuring legends in pop culture and sports in his new music video.