The heat at the Makhulong Stadium in Tembisa made it a tough contest for both teams to get going.
The Absa Premiership log leaders settled quicker, Khama Billiat and Themba Zwane combined in the 8th minute to create the first chance of the game, but the latter’s tame effort was dealt with easily by the Highlands keeper Tapuwa Kapini.
Pitso Motsimane’s men were limited to long range efforts with the Tembisa team happy to let Downs control possession.
Dolly and Teko Modise both tried their luck from outside the box but neither were accurate enough to open the scoring.
The second half followed much in the way of the first.
Downs did the hard running, while the National First Division outfit soaked up the pressure.
Billiat found space on the right wing in the 64th minute, but his cross was too high for Dolly to meet.
The home team then almost stole the win in the 89th minute as Simphiwe Tshabalala flashed a dangerous ball across the Sundowns penalty area, but no Highlands player could get close to it.
The teams could not be separated after 90 minutes and extra time ensued.
It was only in added time that the first real opening of the game came to light.
A long range effort from Mothobi Mvala deflected off Thabo Nthethe and fell fortuitously for Richard Matloga but Onyango was alert to the danger, making a brilliant save down low to keep his clean sheet intact.
The visitors finally broke the deadlock five minutes into the start of extra time.
Dolly collected the ball from Mzikayise Mashaba and let rip with a thunderous left foot shot to register his sixth goal of the season.
“What is important is that we keep winning and that we are through. When it is tough you have to squeeze a goal,” Mosimane said.
“My boys did a good job. It was a good game. We could have won it,” Alan Freese, Highlands Park coach said