Duke coach Mike Krzyzewski couldn't remember the exact moment when he made the switch, or even which game it was. Neither could his players.
According to data from Synergy Sports Technology, the change came on Feb. 11 in an 80-69 win over Georgia Tech. That was when Duke made a nearly exclusive switch from man-to-man to a zone defense. Since that victory, the Blue Devils have played zone 82 percent of the time, compared to 25 percent in the three games that preceded it (two of which were losses).
But the "when" is less important to Krzyzewski than the "why."
His players couldn't stop anyone playing man-to-man. Opposing teams couldn't hold the Blue Devils' high-powered offense in check, but the opposite end of the court had become a nightmare. Guards were beating senior Grayson Allen off the bounce with regularity, wings were driving past Marvin Bagley III, and bigs were making fellow freshman Wendell Carter Jr. look like he was swimming in quicksand.
"It was an embarrassment," one coach who faced Duke told ESPN. "It was a combination [of the fact] that they didn't want to defend, and that they couldn't defend."
Duke's 82-78 loss to rival North Carolina on Feb. 8 marked the 13th game (of the first 24 of the season) in which an opposing team scored more than 70 points; just one of the Blue Devils' opponents has managed to exceed that total in the 11 games since.
"We never declared it," Allen said of the philosophical shift. "It started to work for us. We had a stretch where three or four times teams couldn't score 60 points. I felt like it was our best defense, a really good adjustment for us."
"When we were in man, we weren't really stopping guys," added freshman Gary Trent Jr. "Our rotations were off, and the 2-3 seemed like it was the best thing for us."
It was Aug. 22, 2010, and there were 17 seconds remaining in a Team USA friendly against Spain. The U.S. lead, which had swelled to 13 through three quarters, stood at 86-85.
Syracuse coach Jim Boeheim, then an assistant coach alongside Krzyzewski for Team USA, leaned over and made a suggestion.
"We can't stop them," Boeheim said. "Let's try the zone."
Boeheim figured Krzyzewski would shake his head sideways, but instead he responded with, "Let's do it."
"I'm glad that worked," Boeheim joked. "Because he would have killed me if it hadn't."
Krzyzewski and Boeheim won OIympic gold medals as part of Team USA's coaching staff in 2008, 2012 and 2016. It was during this time that the Duke coach truly honed in on the nuances of the vaunted 2-3 zone that Boeheim has been utilizing for decades at Syracuse.
Krzyzewski, primarily a proponent of man-to-man defense over the years, never anticipated advancing to the Sweet 16 while implementing the 2-3 zone, much less that his next opponent would be Boeheim's Orange on Friday night.
"He got an idea of how to run it and saw some of the things we do," Boeheim said of how Krzyzewski picked it up. "And he's adapted it to his personnel. I think they play it well, not like we do. They play it differently.
"His big guys really make it tough. They are more physical and stronger inside than we are. We're used to playing it, we've practiced more with it. We try and cover the outside a lot more than they do."
On a famously young team -- KenPom ranks Duke 350th out of 351 Division I schools in terms of experience -- few of Duke's current players had even played zone prior to their arrival in Durham.
Keeping four freshman starters in their man-to-man comfort zone would have seemed like a reasonable approach to any coach, much less a Hall of Famer such as Krzyzewski.
At the same time, for all its talent, this team was not consistently functioning in man. Allen, the de facto leader of the team, is a self-proclaimed introvert who was slow to grow into a leadership role and was perhaps not as vocal as he could have been on that end of the floor.
Meanwhile, Duke favored a lineup with a pair of defensively challenged big men, Carter and Bagley, on the court at the same time. While it would be an overstatement to say it was a recipe for disaster -- Duke was 19-5 and No. 9 in the country before it beat Georgia Tech utilizing the zone -- defense was also a worry for a team with a goal of stringing together six straight wins in March.
"We don't talk as well as a team in man," Krzyzewski admitted. "You know, communicate. I think we think too much of our individual assignment. Whereas in zone, we really talk well or we talk better. Overall, our communication is just really at a much different level, and, you know, if you're communicating, you have a better chance to be one. And so that's how we got here, and we're a good defensive team."
Few who watched the Blue Devils in their 81-77 loss to St. John's, for example, would have believed the words "good defensive team" could be synonymous with this squad, but now opposing coaches have sleepless nights preparing for Duke's zone.
In the backcourt, Allen stands at 6-foot-5. While Trevon Duval is just 6-3, he is extremely long and athletic with a 6-9½ wingspan that is difficult on opposing guards.
The back line is also imposing, with Trent (6-6 with a 6-8½ wingspan), the ultra-athletic Bagley (6-11), and Carter (6-10, 7-3 wingspan). Throw in Marques Bolden (6-11, 7-6 wingspan) and Javin DeLaurier (6-10) off the bench, and attacking the Blue Devils becomes extraordinarily difficult.
"You better have big perimeter people that can pass into the high post," Rhode Island coach Dan Hurley said after his team managed just 62 points in a second-round loss to Duke. "Problem becomes even when you get it to the high post, they have that type of rim protection and then they fan out and leave Carter one-on-one protecting the rim and fan out and take away the 3-point line. You're in a world of trouble."
Now it's Krzyzewski vs. Boeheim on Friday in Omaha with an Elite Eight appearance on the line. Zone against zone. In their first meeting this season on Feb. 24, Syracuse saw Duke's metamorphosis up close and personal, when the Orange shot 31.5 percent and committed 17 turnovers in a 60-44 loss in Durham.
"The biggest thing is you need to practice it, or you can't use it in a game," Boeheim said of the 2-3. "His zone's gotten better because they are practicing it all the time. You can't just throw it out there and think it's going to work."
The time and date of the switch won't matter when the ball goes up on Friday. What does matter is that the switch has turned an already great Duke team into a championship-caliber one, and undoubtedly increased its chances of cutting down the nets at the Final Four in San Antonio. If the Blue Devils are to get past the Orange, it will likely be because they applied the zone principles that, if not perfected at Syracuse, were at least refined there.
"We practiced it with the Olympic team, so Mike knows it," Boeheim said. "He has our game film at Syracuse. It's really not that complicated."
Seth Walder from ESPN Analytics provided data research.