SAN FRANCISCO – Down a mere six points less than five minutes into the second half Thursday, Duke coach Mike Krzyzewski knew his team was on the ropes, the end of his legendary career looming ever closer.

The rugged Texas Tech Red Raiders had dictated the tempo throughout the first half of their NCAA Tournament West Regional semifinal and were continuing to impose their will. The Blue Devils were fortunate the deficit wasn’t bigger, but it would surely grow if something didn’t change.

So Krzyzewski switched to a zone, a defensive scheme his players had practiced just a bit during the season and, if not a tactic of last resort, was probably the penultimate one.

The results were not dramatic, but they made enough of a difference that second-seeded Duke was able to escape with a 78-73 victory to give the retiring Krzyzewski his 100th NCAA Tournament win and send the Blue Devils to the Elite Eight for the 17th time in his 42-year tenure.

On Saturday, the second-seeded Blue Devils will face No. 4 Arkansas, which upset top-seeded Gonzaga 74-68, with a spot in the Final Four on the line. If they advance that far, Krzyzewski would set a record with a 13th Final Four appearance, breaking a tie with the iconic John Wooden of UCLA.

“It kept down the amount of physicality because they were wearing us down, so the zone gave us a chance to kind of dance around the ring a little bit instead of being in a corner,’’ Krzyzewski said.

A 9-1 surge gave Duke its first lead of the second half, 49-47, when Paolo Banchero (game-high 22 points) hit a short baseline jumper at the 11:35 mark. The teams were not separated by more than four points until the Blue Devils opened a 73-68 lead with 1:30 left to play, then iced the game at the free throw line.

Duke made its last eight field goal attempts as it became only the third team this season to score 70 points against the Red Raiders, who rank at or near the top in most national defensive metrics.

The Blue Devils played zone for only about seven minutes, and it wasn’t all that effective until they tightened it enough to keep third-seeded Texas Tech from beating them with cuts to the basket.

The defensive changes illustrated not only the dire nature of the moment but also Krzyzewski’s willingness to adjust and trust his players. When they later asked to go back to man-to-man, he complied, saying the request came from the whole group, “like a Catholic boys’ choir. It was a chorus. They all said it.’’