The Horned Frogs felt they had to play a near-perfect game to win on the Aztecs' homecourt but didn't come close to meeting that goal and committed 16 turnovers in the 83-73 loss.
The loss leaves TCU (13-9, 3-4) in sixth place in the Mountain West heading into the second half of play.
"There's not one thing, it's a bunch of things," upset coach Jim Christian said after the loss. "It's disciplined stuff. We're just not being consistent and disciplined, especially on the defensive end of the floor. You're not going to be able to beat them."
The Horned Frogs played well early and even had a seven-point lead before the self-destructing sequences began to catch up with them.
"We just did not play a very good game," point guard Hank Thorns said. "We had too many mistakes and it's hard to beat a good team like San Diego State when you play like that.
"It's hard to know how to fix it. We were trying, but nothing seemed to work."
The Horned Frogs are four games over .500 entering a nonconference contest against Southern Methodist on Feb. 8. Winning that contest would bolster TCU's chances at finishing with a winning record for the first time in Christian's four seasons as coach.
--TCU is 10-2 at home this season and won all three of its Mountain West home games in the first half of conference play. Although the Horned Frogs don't have the boisterous crowds backing them that many of the other Mountain West school do, it is certainly becoming harder for opposing teams to win in Fort Worth.
--TCU connected on 12 3-pointers against San Diego State to match a season-best output first accomplished against Colorado State on Jan. 14.
--The Aztecs shot 50.8 from the field to become the 10th team to shoot 50 percent or better against the Horned Frogs this season.
BY THE NUMBERS: 5 -- TCU's home winning streak entering its Feb. 11 contest against Colorado State.
FUTURES MARKET: Rebounding again was an issue in the loss to San Diego State. The Horned Frogs were out-boarded 35-21 and struggle to mix it up inside with Mountain West foes. In seven conference games, TCU has been outrebounded by an average of 7.1 per game. Guard J.R. Cadot has a team-best 6.4 rebounding average and is averaging 5.0 boards in conference play. Cadot and forward Garlon Green (4.1 in league play) are the only Horned Frogs average more than four boards in league play. Rebounding must improve for TCU to flirt with a .500 conference record.
--Senior PG Hank Thorns ranks ninth in school history with 324 assists after passing former teammate Ronnie Moss (323) with four assists against San Diego State.
--Freshman G Kyan Anderson scored a career-best 22 points against San Diego State for his second 20-point outing of the season.
--Senior G J.R. Cadot made a career-best three 3-pointers against San Diego State.