Cemaes moved to within two points of Premier leaders Bethesda after stretching their unbeaten run in all competitions to seven matches on Friday night.
Bay were three-up on Llanberis by half time, Tom Wood with a double before new signing Cameron Buckley added a third.
The Darans had Alan Mark Owen sent off for a second yellow in the 34th minute.
The Anglesey hosts went four ahead in the 88th minute through 17-year-old substitute Codie Holland.
Arwyn Jones (2) and captain Kyle Williams delivered the goals in a fine win for Llanystumdwy at Treborth.
This tie stood at 0-0 until the 89th minute before Nefyn turned on the power in the closing stages.
Llanystumdwy were down to 10 men after Iwan Richards was shown a second yellow card on 88 minutes before soon after Josh Piper broke the deadlock.
Nefyn made sure of a quarter-final berth after Tomi Evans netted on 91 and 99 minutes.
Cian Pritchard delivered a double and Gwion Davies was also on the mark as Penrhyn moved into the last eight.
Tom Wood (2) and Cameron Buckley shared the goals as Cemaes took maximum points at Cae Cynlas, Dion Billing having handed Aberffraw a 13th minute lead.
Iwan Jones pocketed a 45th minute winner for Blaenau, who remain top of the table.
Jerome Sillence was a four-goal hero for third-place Caergybi, Steve Driedyk adding a pair and sub Owain Griffiths also making the scoresheet, Luke Pumfrey with Cefni’s sole reply.
Sion Foulkes’ hat-trick – which included a penalty – inspired Deiniolen to a convincing win over visitors Llangoed.
Meurig Parry also found the net for the hosts, as well as adding two assists.
Jay Finter, Dylan Jones and Calum Thomas came up with the decisive goals for Gaerwen, Dan Jones replying with a penalty for Amlwch, who had Martin Byast red-carded on 25 minutes.
Captain Ben Davies scored the only goal of the match on 61 minutes to ensure Rangers made it back-to-back league wins.
Quite a surprising scoreline in this semi-final with Llanfairfechan, 10th in tier 4 North Wales Coast East Premier, hammering Bethesda, top of the North Wales Coast West Premier.
Elliott Croston (2), Harry Croston, Ellis Jones and Harry Parkes-Smith split the goals for impressive Llanfair, who will meet wither Gwalchmai or Penmaenmawr Phoenix in the final.
Pesda were under pressure from early on after key defender Shaun Lock was red-carded in the fifth minute at the Recreation Ground.
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