The Egyptians started the better of the two sides, looking more of the attacking threat, yet it was Downs who had the first shot on target, Hlompho Kekana letting fly from range to force a diving save from the Zamalek shot-stopper.
Zamalek continued pressing forward, with Downs lucky not to go a goal behind after Wayne Arendse’s missed clearance gifted the Egyptians a shot on goal, which could only be lofted over the bar by Ayman Hefni.
The Brazilians took the lead soon after however, as Zamalek were dispossessed in midfield, Leonardo Castro feeding Khama Billiat whose well-taken shot bounced off the post, yet Tiyani Mabunda was on hand to smartly finish for his first official Champions League goal.
The goal spurred Zamalek on as they raised their game, and following a period of sustained pressure, levelled matters following a calamitous error at the back: a looping ball was not dealt with as Denis Onyango and Thabo Nthethe both went for it, but Mohamed Ibrahim was quickest as he snuck inbetween the duo, lofting the ball home into an empty net to draw level.
Castro’s shot late in the half was blasted wide as the two sides headed into the break on level terms, 1-1.
A near mishap at the back early in the second half almost gave the hosts the lead as Onyango failed to control a simple back-pass, as he hastily cleared the ball off the line.
Goal-scorer Ibrahim then had his evening brutally cut short after a strong Nthethe tackle which left the Egyptian with what looked like a broken ankle as he was carted off the pitch.
Downs continued to probe, with substitute Percy Tau injecting more pace into the attack as Pitso Mosimane’s men edged closer to another goal – Castro headed a Billiat free-kick into the arms of the Zamalek keeper, before Billiat himself had a shot on goal, controlling the ball superbly before producing a fine save from El-Shenaway in the Egyptian’s goal.
Castro then acrobatically hit the crossbar after fine control in the opponent’s box, before Billiat finally found his reward, collecting a fine pass from Keagan Dolly before expertly finishing with his left foot in the bottom corner to restore his side’s lead.
Onyango was on high alert for the remainder of the match, punching away a fine swerving shot late on before rushing off his line to snuff out a dangerous through-ball three minutes from time.
Tau blasted a glorious chance wide on the stroke of full-time, as Downs were made to see out a heart-racing five minutes of added time before the final whistle.
The visitors however held on for a tough 2-1 win in Egypt, leaving them with six points in Group B, three points ahead of Zamalek, and with one foot in the semi-finals.
They next host the very same Zamalek at home in two weeks’ time.
Zamalek XI: El-Shenaway, Gomaa, Koffi, Gabr, Emam, Hamed, Tawfik, Emam, Hefny, Ibrahim, Shikabala, Morsy
Sundowns XI: D. Onyango, S. Zwane, T. Nthethe, W. Arendse, T. Langerman, T. Mabunda, H. Kekana, A. Laffor (Tau 57'), K. Dolly, K. Billiat, L. Castro (Mbekile 93')
Unused Subs: Sandilands, T. Zwane, Bangaly, Malajila, Modise