The Western Australian bagged four cheap wickets and top scored at No 7 as his side claimed a three-wicket win at Allotment Road.
DeGrussa then struck twice in quick succession, dismissing Sri Lankan overseas Minhaj Jalill (2) and James Christian (0).
And the visitors lurched to 54-4 when Dan Wimble (4) became DeGrussa’s third victim after Duggan had elected to bat first.
Opener Jack Marson (28) hung around until he was eighth out with the score on 132.
Henry Woolf (22) and Joe Smitherman (17) added late order runs before DeGrussa ended with 4-16 when he trapped Reuben McArdle leg before to end the Portsmouth innings on 145.
DeGrussa’s second major contribution came when he walked out the middle with Sarisbury in big trouble at 66-5.
McArdle (2-23) had removed Sam Floyd (11) and captain Josh Hill (0) while Jalill (2-36) dismissed opener Tom Mills (36) and Ricky Rawlins (7).
It was only DeGrussa’s third innings for Sarisbury in SPL action, with his previous two scores being 1 not out and 3 not out. For his home club Gosnells in Western Australia, he had averaged just 7.23 in 2021/22 with a top score of 23.
But he helped rebuild Sarisbury’s innings, putting on 46 for the sixth wicket with Nathan Feltham (34 off 35 balls).
De Grussa added another 24 for the seventh wicket with Harris (11) as Sarisbury approached their target.
He was still there at the end, unbeaten on 37 off 65 balls - including five fours - as the hosts won in the 44th over.
Sarisbury and Portsmouth are now two of five teams who have won three and lost two of their opening five matches, and are separated by just three points.
Sarisbury are fourth, two places above Portsmouth, with Rowledge clear at the top having won four games out of five.
McArdle, meanwhile, is the leading wicket-taker in the second tier of the SPL with 13 at 12.92 - two ahead of DeGrussa (11 at 14.45) and Liam Carty of Calmore (11 at 14.18).
Harris, with 10 at 20.70, is the joint third highest wicket-taker in his first season in Division 1 after stepping up from Division 3 outfit Gosport Borough.
Duggan, with 271 at 90.33, is the second highest runscorer in the division. Top of that chart is Ventnor’s Sri Lankan Dineth Thimodya, who struck his second SPL ton of the season in the loss against Alton at the weekend.
Having hit 151 on his second Ventnor SPL outing against Sparsholt, Thimodya hit an unbeaten 100 in Ventnor’s 210-8 - but Alton won by seven wickets.
Thimodya - who hit an unbeaten 103 on his Ventnor debut in a pre-season friendly - now has 293 Division 1 runs at 97.67.
*Ollie Baker bagged a remarkable 3-4 off 10 overs as second tier leaders Rowledge recovered from a dismal start to beat Calmore by 19 runs.
Rowledge were a sickly 93-7 before No 8 Ryan Littlewood (48) helped them post 174-9, adding 63 for the ninth wicket with Richard Forbes (18 not out).