In Potchefstroom, contrasting centuries from Divan van Wyk and David Miller had bolstered the Dolphins to 373 for six declared, before right-arm quick Alexander bagged three for 23 to leave the Highveld Lions on 73 for six in pursuit of 501.
It capped another domineering day for the Durbanites, who had resumed on 70 for two at the start.
Van Wyk recorded his 12th career century from 214 balls, before falling for 114, while Proteas limited-overs star Miller followed up his first innings 109 with 104 in 140 deliveries -- his fourth career ton and the first time he had hit three figures twice in the same game.
That allowed Dolphins captain Morne van Wyk to declare and the Lions' top order was then blown away by Alexander, who dismissed Stephen Cook (one), Quinton de Kock and Dominic Hendricks (both ducks) to leave the total on seven for three.
Mthokozisi Shezi (2/20) also joined the party for the visitors not long after by capturing the key wickets of Temba Bavuma (22) and Neil McKenzie (20), while Graham Onions had Zander de Bruyn caught behind for a duck.
That left the Johannesburg outfit on the brink of a heavy defeat, a massive 428 adrift by the close with just four wickets remaining.
Meanwhile in Port Elizabeth, Parnell claimed three for 24 to leave the Titans on 84 for four in pursuit of 440 after the Warriors had declared their second innings earlier on 183 for seven.
The Pretoria visitors had began the day in some bother on 180 for six in reply to the home side's 502 and they managed to add a further 66, thanks mainly to an unbeaten 64 by Roelof van der Merwe before they were all out for 246 -- a deficit of 256.
Parnell was again the pick of the bowlers ending with three for 46.
The hosts then came out attacking in pursuit of quick runs, and although they lost plenty of wickets, half-centuries by Jon-Jon Smuts (52) and Ashwell Prince (62) ensured they only needed to bat for 42.1 overs before they could declare.
Rowan Richards ended with three for 35 for the Titans, who then lost Heino Kuhn (four), Jacques Rudolph (nought) and Henry Davids (three) to Parnell, slipping to 18 for three in the process.
Simon Harmer added to the Titans' woes by removing David Wiese (four) before the close, which left the Eastern Cape side in complete command.
Finally in Cape Town, a sixth first-class century from Pite van Biljon helped the Knights bat themselves into a 40 run lead, before the Cape Cobras reached the close on 74 for two with the match seemingly heading towards a draw.
All-rounder Van Biljon struck 112 from 340 balls at Newlands to help the Knights, who resumed on 187 for four overnight, to post 355.
Werner Coetsee also made a useful 46, while the competition's top wicket-taker bagged four for 69. Rory Kleinveldt also finished with three for 61.
Any hopes of a victory for the Free Staters were dashed when the hosts batted sensibly in their second innings, losing just Omphile Ramela (25) and Stiaan van Zyl (12), the number three batsman falling from the last ball of the day, with the lead 34.