Sergis had earlier sent a scare through the Roosters camp when she twisted awkwardly in a tackle late in the first half by Ngatokotoru Arakua.
She needed her right ankle strapped but stayed on the field. In the second half she appeared to hobble around at times, but played out the full game in a testament to her courage.
Referee Kara-Lee Nolan placed the Knights forward on report because her shoulder had made contact with Sergis' head.
Then minutes later Arakua was placed on report for a second time - this time for lifting Keilee Joseph - and ended up in the sin bin.
It meant the Knights started the second half with 12 players, which in turn led to two quick tries by the home side.
With both teams losing their opening two rounds, only one outfit was destined to go home happy.
Going into the match, Newcastle rebooted their halves pairing with Charntay Poko and Kirra Dibb coming in at No.7 and No.6.
Autumn-Rain Stephens-Daly moved to the wing and regular halfback Tahlulah Tillett was named on the bench.
The Roosters had the bulk of the possession early so no surprises they were the first to post points after just seven minutes.
It came through a long cut-out pass from halfback Raecene McGregor to link with backrower Yasmin Meakes. The ball merely passed through the hands of Isabelle Kelly to put winger Brydie Parker over in the south-east corner untouched.
McGregor was at it again just minutes later to create more space with a long-range ball.
However, this time it was Parker passing inside to Kelly for the Roosters to go back-to-back and scoot to 10-0 after 13 minutes with the Hannah Southwell conversion.
The Knights peeled one back when fullback Romy Teitzel poked her nose through in the 24th minute, dragging three defenders with her. A Kirra Dibb conversion and the contest was far less one-sided at 10-6 at the break.
In the second half with Arakua still in the bin, Parker got her double before second rower Olivia Kernick got a favourable bounce from a charge-down to score the Roosters' fourth try.
Kiwi Ferns prop Annetta Nu'uausala barged over with 19 minutes left on the clock and it was a six-point game (18-12).
But two Knights errors close to their line led to two tries - to Joseph under the posts and Sergis out wide - and the gap widened with four minutes left.
In other injuries, the Roosters' NSW Origin forward Sarah Togatuki had a cut above her eye and didn't return after failing her HIA.
