Defending NRL premiers Sydney Roosters let an early lead slip to lose their 2020 opener against Penrith Panthers on Saturday.

Penrith produced a fine second-half performance to beat the Roosters 20-14, with Viliame Kikau - battling a shoulder injury - and Api Koroisau particularly impressive.

Sydney - premiers in 2018 and 2019 - had breezed into a 12-0 lead inside 20 minutes, with tries from Mitchell Aubusson and Siosiua Taukeiaho and a penalty from Kyle Flanagan, but Brent Naden went over after 26 minutes to begin the Panthers' fightback.

Stephen Crichton's try helped level the scores at 14-14 with 16 minutes left on the clock, and Nathan Cleary - whose exemplary kicking bolstered Penrith throughout - provided the pass from which Kikau scored a 69th-minute winner.

Elsewhere, Newcastle Knights made a winning start to the season under new coach Adam O'Brien, whose side beat New Zealand Warriors 20-0 at McDonald Jones Stadium.

O'Brien's positive impact on the Knights defence was in evidence throughout a one-sided encounter that saw Daniel Saifiti open the scoring with an eighth-minute try that Kalyn Ponga converted before Edrick Lee went over to make it 10-0 at the interval.

Ponga scored all of the Knights' second-half points, kicking two penalties in six minutes before landing a 54th-minute try that he converted himself to give his side an unassailable lead.

South Sydney Rabbitohs had to scrap harder for their first win of the new campaign at ANZ Stadium, where captain Adam Reynolds' kicking made the difference in a 22-18 victory over Cronulla Sharks.

After the Rabbitohs opened up a 22-8 lead with 20 minutes left to play, courtesy of first-half tries from Reynolds and Campbell Graham along with a pair of Reynolds penalties and Ethan Lowe's second-half score, the Sharks launched a late fightback.

Sione Katoa went over twice in three minutes to narrow the deficit to four points, before Connor Tracey looked to have sent Josh Morris through for a try.

But Tracey's pass was ruled forward to the relief of Rabbitohs coach Wayne Bennett, whose side took the points.