Lagman played steadily and put-up a good showing in the 12 and 14-under classes to proceed to the penultimate round.
In the 12-under, the top-ranked Lagman defeated Al-Khady Jainul, 6-2, 6-3 to meet Angelo Joseph Rivero next.
In the 14-under, Lagman continued to make upset wins, defeating top-seeded players in his first attempt.
Lagman, who turned 11 last month, first upset seeded no.3 Princeton Gerapusco, 7-6, 2-6 (10-8).
Yesterday, another seeded netter, Joseph Balbona bowed down to Lagman in another thrilling match, 1-6, 6-3 (10-4).
With the victory, Lagman meets top-seeded Kyle Cordero next.
Jacob's brother, Julius, on the other hand, won against Tim Polero, 6-4, 2-6 (10-4) in the 16-under and then pulled a 7-5, 6-2 win against Fidel Regis.
However, Julius Lagman failed to sustain his winning ways and lost out of contention after losing in his assignments yesterday.