Western Australia 2 for 192 (Bancroft 106*) trail South Australia 8 for 510 dec (Head 223, Hunt 10) by 318 runs
Cameron Bancroft scored his second Sheffield Shield century of the season to lead a strong Western Australia reply to a huge South Australia total based around Travis Head's maiden first-class double hundred.
Bancroft, who made the most of a life on 13, reached his hundred shortly before the close as he and Cameron Green guided the home side into a position from where they will feel comfortable avoiding any potential follow-on then closing in on the Redbacks' score.
The third-wicket stand was worth 120 at stumps and ensured Western Australia did not suffer further setbacks after Sam Whiteman was caught at slip off a beauty from Dan Worrall then Shaun Marsh clipped a full toss from the part-time offspin of Will Bosisto to midwicket on the stroke of tea. That would be Bosisto's only over of the day.
South Australia's pace attack toiled hard during the final session on a surface that has not offered much for the seamers. Green, as is his style, took time to get his innings up and running - he was 2 off 31 balls early on - but had collected seven boundaries by the end of the day.
South Australia had resumed handsomely placed on 4 for 368 and Head went to his double hundred in the second over of the day with consecutive boundaries off Liam Guthrie. His 245-ball innings was eventually ended when he was superbly caught at slip by Marsh.
The visitor were taken beyond 500 by Worrall and Jake Lehmann, the former hitting 39 off 41 balls before his dismissal brought the declaration.