Mennie took two quick wickets in the first session to have WA 2/37 after South Australia lost the toss and were sent into the field in sunny Perth.
Swing bowler Sayers then struck twice in the middle session, claiming Shaun Marsh for 41 with a sharp caught and bowled effort before picking up Warriors captain Ashton Turner for 4 in his next over.
Sayers, 32, joined an elite group in the process by becoming just the fifth player to claim 250 Sheffield Shield wickets for South Australia.
WA hit back from 4/94 through a 122-run stand shared by Marcus Stoinis (79) and Cameron Green, who remains unbeaten on 95* at stumps.
Josh Inglis (42*) will resume alongside Green on Saturday with the hosts 5/296 at the conclusion of the Day One.
Promising 18-year-old debutant Liam Scott was presented Redbacks cap No.640 by former SA opening batsman David Fitzgerald prior to the day’s play at the WACA.