The teenager had started the 2021 season in the St Cross 1st XI in the top flight - but made scores of 5 and 0 in his first two games.
After dropping down to the 2nds, Trussler followed up his 75 against Sparsholt with a career best 144 not out at St Helens.
It is the highest innings recorded anywhere in the four divisions of the SPL so far in 2021.
After former Hampshire batsman Jason Laney had won the toss and elected to bat, Trussler joined his skipper early on after Steven Hirst had been bowled by Henry Woolf for a single.
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The pair proceeded to add 121 for the second wicket before Laney was run out by Will Smitherman one run short of his half-century.
Trussler, who hit 102 for St Cross 1sts against Andover last year, went on to smack 18 fours and three sixes in his 120-ball innings as St Cross posted 238-4 - Charlie Harden (20 not out) helping him add an unbroken 69 for the fifth wicket.
Woolf, who took 1-27 in eight overs, was Portsmouth’s most economical bowler.
Skipper James Christian had scored an unbeaten 104 as an opener in Portsmouth’s win at Basingstoke the previous best.
But this time he dropped back to No 3 to allow Jack Marson to return to the top of the order alongside Ben Duggan.
Marston was to top score with 35 but, with Duggan and Christian both out for 10, Portsmouth sank to 74-3.
With South African Cameron Braans bagging 4-16 off eight overs, the hosts slithered into deeper trouble at 104-6.
With Alex Hammond (15) the only player outside the top four to reach double figures, Portsmouth were eventually dismissed for 132 - 12 runs fewer than Trussler had earlier managed on his own.
The city club’s 106-run loss leaves them third bottom after six rounds of games.