The 17-year-old came off the bench and moved into his favoured attacking midfield role after starting the second half on the left, capping a 4-2 penalty shootout win over Sydney FC with a nerveless Panenka penalty in the shootout that followed a 1-1 draw at Allianz Stadium, where Mathys Tel's free-kick had been cancelled out by Takahiro Sekine.
Sandro Tonali, debutant Andy Robertson and Dane Scarlett converted the decisive spot-kicks after goalkeeper Martin Dubravka saved from Alexander Popovic.
"We have to manage him like a precious diamond," De Zerbi said of Williams-Barnett after the match. "We have to help him to improve in the last decision. Physically, work harder to become stronger because this time of football, he needs to be strong, and to give him space when there is the right moment.
"I usually give time to all the young players I work with in my career, and he is a potential special player."
It continues a run of eye-catching performances for the academy graduate, who impressed in a deeper role during last week's 1-0 behind-closed-doors win over MK Dons and played an hour in Sunday's 2-0 victory over Auckland FC, making the most of an unusually open opportunity created by Tottenham's World Cup absentees, injuries and a lack of match fitness across the senior squad.
Williams-Barnett, who joined Spurs at Under-10 level, made his senior competitive debut last September aged just 16 years, 11 months and 24 days, when introduced by then-manager Thomas Frank in the closing stages of a Carabao Cup third-round tie against Doncaster Rovers — making him the youngest player in the club's history to feature in the competition.
Tottenham defender Ben Davies also praised the teenager's temperament following the win in Sydney.
"He's a really top player," Davies said. "He's got a lot of potential. I think you can see the qualities that he's got. I think now the next step is developing his habits in the game. But to have a player like that who's got no fear, that has the confidence to try things in games, who doesn't get affected if he loses the ball, I think he's got a bright future ahead of him."
The win made it three consecutive pre-season victories for Tottenham under De Zerbi as they build towards the opening weekend of the Premier League season away to Brentford on August 22.