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Rossouw, making just his second Proteas appearance after ending his six-year Kolpak exile, hit five sixes and 10 fours and looked certain to become just the fifth South African to reach three figures in Twenty20 cricket.
Jordan allowed just four runs off the 20th over and finished with a dot ball when Rossouw needed a boundary to make his landmark.
England will still need to fire to complete a stiff chase and claim an unassailable 2-0 lead in the series but, having scored 234 for six at Bristol on Wednesday, they know they have the firepower.
South Africa suffered an early setback when a Jos Buttler hunch paid off, Moeen Ali thrown into the attack in just the fourth over and almost immediately persuading dangerman Quinton de Kock to pick out Jason Roy at mid-off.
But they continued to score freely, reaching 58 for one in the powerplay and 100 at the halfway point.
Reeza Hendricks made his second accomplished half-century in as many days, taking just 29 balls, while Rossouw was in fine touch. He smacked Adil Rashid out of the ground and towards the River Taff and lashed Jordan back down the ground twice in his first visit.
Jordan thought he had the number three caught behind on 37 but DRS raised doubts over Buttler’s catch and Rossouw was freed up to continue his assault.
Hendricks, who hit two sixes and three fours, was dropped by Richard Gleeson on 51 only for the pace bowler to make amends in the next over.
Inviting the pull, he had Hendricks well caught by Jonny Bairstow, who watched the ball all the way to the rope at long-leg and stopped dead in his tracks as soon as he held it to avoid careering over.
That ended a stand of 73 in 46 balls, with Jordan returning to pick up Heinrich Klaasen, whose under-powered drive hung in the air on its way to Dawid Malan.
Rossouw carried on after ramping Gleeson into his own jaw on 72 and joined Tristan Stubbs to tick the total beyond 200.
He looked to have timed his own surge to a century perfectly but Jordan stymied his efforts with an excellent concluding over.