Spurs broke their transfer record twice in the space of two days last month, paying West Ham £85m for Fernandes before eclipsing that fee with a £100m deal to sign Tonali from Newcastle — a combined outlay of £185m that took many by surprise given both players reportedly had other options on the table.
Tottenham narrowly avoided relegation to the Championship on the final day of last season, making the scale of the spending all the more striking.
De Zerbi credited his own understanding of what makes players tick for helping to get both deals over the line.
"To want to convince one player, I have to be able to come inside their head," he said. "Tonali was very young when he started to play in Brescia and was a leader, and I think Tonali needs to play in a big club with big motivation. For Mateus Fernandes, it is a crucial part of his career because he needs a particular style of play to show all his qualities."
The Italian also spoke warmly of Jan Paul Van Hecke, who joined for £55m from Brighton and is set to link up with the rest of the Tottenham squad imminently.
"Van Hecke is like my son because I worked with him for two seasons at Brighton. I know him very well as a character, a personality, as a leader," De Zerbi said.
Fernandes, Tonali and Van Hecke form part of six summer signings so far at Tottenham, alongside free transfers for Andy Robertson and Martin Dubravka and the arrival of Marcos Senesi, as De Zerbi looks to overhaul a squad that struggled for large parts of last season.
The combined spending on Fernandes, Tonali and Van Hecke alone takes Tottenham's total outlay for the window past £237m, surpassing the previous club record of £225m set in the summer of 2023, and there is no suggestion the club's business is finished with the market still open for several more weeks.
Both Fernandes and Tonali had been strongly linked with other clubs before their moves to north London, making De Zerbi's personal involvement in the negotiations all the more significant in the eyes of those at the club.
Spurs returned to pre-season training two weeks ago and play their first friendly of the summer against MK Dons at Hotspur Way on Wednesday evening, with De Zerbi expected to run the rule over his new-look squad for the first time.