Everything seemed to go Ahly's way when they went 5-1 up five minutes past the hour mark as a victory similar to the one the Red Devils recorded over their bitter foes in 2002 seemed well on the cards.
But they needlessly retreated into their own half and Zamalek took advantage to give the scoreline a more respectable look, with winger Ahmed Zizo converting an 80th-minute penalty and Moroccan winger Achraf Bencharki drilling home in the final minute.
Mosimane was asked by reporters whether Ahly could have added more goals, but he played down the significance of Zamalek's late rally.
"We scored three goals in the first half and two in the second half. I wanted more, but we can't have everything," the South African coach, who led Ahly to back-to-back African Champions League crowns but was still criticised when he lost last season's league title to Zamalek, told the post-match news conference.
"We scored five goals. Maybe in Ahly's history against Zamalek the team scored more goals," he added, before asking the reporters about the last time the team registered a big derby win.
When he was told that Ahly hammered Zamalek 6-1 in a league game in 2002, Mosimane replied: "Good days are back."
Friday's 5-3 win marked the highest-scoring Cairo derby ever, eclipsing Ahly's 4-3 victory over Zamalek in the Egypt Cup final in 2007.
Ahly made it three wins out of three league games this season, moving level on nine points with Pyramids FC, the only other team to bag maximum points.
"We got a good result, my players played with good spirit, we were tactically clever and we were a little bit smarter. We could find holes to move the ball inside," Mosimane added.
"We scored five goals in a big game. I'm very happy with that."
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