The reigning champions looked on course to defend their title after soaring to the summit upon catching up their games in hand which at one point had Zamalek 19 points ahead of them.
However, their 1-1 draw against Ismaily has seen them drop two points off Patrice Carteron's side with five games remaining in the 2020/21 campaign.
Veteran Tunisia forward Fakhereedine Ben Youssef found the net just after the hour mark to cancel out Mohamed Sherif's first-half opener which seemingly defied Carteron's recent statements that Ahly's run of fixtures are like 'friendlies'.
Mosimane conceded that the result is a massive blow but refused to throw in the towel and denied suggestions that pressure or complacency crept in after a run of impressive form earned them the title-favourites tag again.
"Well I'm not sure about that, I can't guarantee that, I'm not sure. It's very difficult for me to say that because we weren't nervous," he said in the post-match press conference.
"The title isn't finished yet. We have to focus and believe because we don't know what's going to happen after.
"Yes, we feel sad because we lost the two points, we know it's a big impact but football isn't like that you have to play until the last minute."
The Club of the Century take on ENPPI next in the league while the White Knights are scheduled to face Ismaily as the riveting title race looks to go down to the wire.