Maessen scored 12 of the visitors’ 18 first-half points.
The remaining four starters for UCI finished with a combined seven points to help produce the second-smallest scoring output this season.
UOP (9-7, 3-1) had three players score in double figures, led by Janae Young, who had 16 points and 13 rebounds.
Big West Conference
Pacific 61, UC Irvine 37
UCI – Belen 0, Barnes 2, Maessen 19, Ishizaki 3, Ke. Johnson 2, Petrovic 6, Ka. Johnson 5.
3-pt. goals – Ishizaki 1, Ka. Johnson 1.
Fouled out – Belen.
Technicals – Team 1.
UOP – Young 16, Simmons 12, Price 3, Tiernan 14, VanHollebeke 5, Fath 5, Dawkins 5, McLeod 1.
3-pt. goals – Tiernan 4, VanHollebeke 1.
Fouled out – Young.
Halftime – UOP, 29-18.