Asked to bat, Abahani Limited rode on half-centuries from Tamim Iqbal, the captain, Nazmul Hossain Shanto, and Mosaddek Hossain Saikat to put up 255 for 6 in 50 overs at the Shere Bangla National Stadium in Mirpur.
Rupganj's chase was halted by rain and thunder, with them requiring 11 runs from 3.2 overs. The umpires subsequently held discussions with both sides, before it was declared that Rupganj were 21 runs ahead of the Duckworth-Lewis par score. It was Rupganj's third win, while Abahani lost for the second time, on both occasions in Mirpur.
Rupganj were jolted early in their chase when Junaid Siddique edged Taskin Ahmed to the wicketkeeper in the sixth over. Soumya and Mithun, the wicketkeeper-batsman, rebuilt through a second-wicket stand of 127.
Mithun was dismissed by Mosaddek in the 27th over, having made a run-a-ball 75 with six fours and three sixes.
Soumya added another 42 for the third wicket with Asif Ahmed and made his first fifty in the competition, but fell for 84, with 76 runs needed. He struck two sixes - over long-on and midwicket - and nine fours in his 89-ball knock.
Abahani came back into the game when they removed Asif in the 41st over, but Rupganj's nerves were eased through the 38-run fifth-wicket stand between captain Mosharraf Hossain and Nahidul Islam, who made 25 each.
For Abahani, left-arm spinner Saqlain Sajib took 2 for 34, while Shanto, Taskin and Saikat took a wicket apiece.
Earlier, Shanto and Saikat, the two most exciting young talents in the country, came in handy with the bat too, adding 125 for the fourth wicket after Abahani lost their way following a good start. Shanto top-scored with 72 off 82 balls that had six sweetly-timed fours and a six, while Saikat was unbeaten on 67 off 75 balls with four fours and a six. But the pair couldn't up the ante towards the end, as Abahani scored only 77 runs in the last ten overs despite having seven wickets in hand.
The duo came together after Tamim had struck his third fifty in the competition, reaching the landmark with a beautifully-struck six over cover. But after driving Taijul Islam through covers, the left-arm spinner got the better of him next ball and cleaned him up. Tamim had made 54 off 64 balls with seven fours and a six.
At the Khan Saheb Osman Ali Stadium in Fatullah, Prime Doleshwar Sporting Club suffered a 59-run loss to Victoria Sporting Club. It was the first defeat in the competition for Prime Doleshwar, who had won all their four matches previously.
Sent in to bat, Mominul Haque struck his second List-A century to guide Victoria to 270 for 3 in 50 overs. In reply, Prime Doleshwar were cruising at 161 for 3 before losing seven wickets for 50 runs to be bundled out for 211. Left-arm spinner Chaturanga de Silva did the bulk of the damage with 5 for 36 - his second consecutive five-wicket haul.
Mominul shared a 162-run second wicket stand with Abdul Mazid, who missed his century by just six runs. Mominul struck a dozen fours and two sixes in his run-a-ball 104, while Mazid hit seven fours and a six in his more sedate 137-ball knock.
After Mominul's dismissal, Al-Amin added 84 for the third wicket with Mazid. Al-Amin smashed an unbeaten 51 off 46 balls with the help of three fours and two sixes. Chaturanga smacked two sixes in his 6-ball 16 towards the end to boost the total.
Al-Amin Hossain, the right-arm medium pacer, took all three wickets that fell, finishing with 3 for 57 off 10 overs.
In reply, Prime Doleshwar lost Rony Talukdar early, before Imtiaz Hossain and Raqibul Hasan added 83 runs for the second wicket. Victoria bounced back with two quick strikes when Chaturanga removed Imtiaz and Chamara Silva in the same over. Thereafter, Nasir Hossain joined Raqibul in a 68-run fourth-wicket stand to get Doleshwar back on track. But they crashed from that point, slipping from 161 for 3 to 169 for 7 in less than five overs.
Chaturanga took his third List A five-for to follow up with his 6 for 35 in Victoria's previous game, against Abahani.
In another rain-interrupted game, Brothers Union beat Prime Bank Cricket Club by seven runs via the Duckworth-Lewis method.
Batting first, Prime Bank were bowled out for 249 in 49.3 overs at the BKSP-3 ground, Shehan Jayasuriya top-scoring with 63.
In reply, Brothers Union were 193 for 3 in 42.5 overs - seven runs ahead of the D/L par score - when rain stopped play. Shahriar Nafees was unbeaten on 84 off 126 balls with eight fours and a six. He had added 74 runs for the second wicket with Tushar Imran, the captain, and another 76 runs for the third wicket with 18-year old wicketkeeper-batsman Zakir Hasan.
Mohammad Isam is ESPNcricinfo's Bangladesh correspondent. @isam84