Pirates started brightly, with former Kabras Sugar star Kelly Akengo slotting two early penalties for a 6-0 lead.
However, Kisumu found their rhythm, with Victor Otieno finishing a well-worked move for their first try, though the conversion was missed.
Gibson Perez added the bonus-point try after patient build-up play, and Dennis Makanji capitalized on a charge-down to extend the lead further. Former Koyonzo High School standout Branton Lusisa capped off the victory with a debut try, sealing the emphatic 38-9 win.
Saturday's Kenya Cup Results
KCB 77-22 Mwamba
Kabras Sugar 45-5 Strathmore Leos
Kenya Harlequin 48-12 Impala RFC
Nondies 23-3 Blak Blad
Menengai Oilers 32-12 Nakuru RFC
Kisumu RFC 38-9 South Coast Pirates