Eleven riders went clear in the first round, only Olympian Katie Laurie achieving the feat on two mounts, Carlotta and Dunstan Springfield.
Cubitt was the third into the ring and the first to go clear. She faced a nervous wait, with three riders then going clear, but none bettering her time. Laurie was closest with 44.07 riding Horse of the Year Champion Dunstan Springfield.
Mills Reef Classic Results: 1, Lisa Cubitt, Bates Amaretto 42.91s; 2, Katie Laurie, Dunstan Springfield 44.07s; 3, Olivia Robertson Ngahiwi Cisco 48.11s; 4, Rose Alfeld, My Super Nova 48.86s; 5, Mathew Gilmour, Cuba D Esqueimes Z 45.55s, 4 penalties.