Cameron Green struck a timely half-century to put Western Australia in the box seat of their Sheffield Shield clash with South Australia at the WACA Ground.In reply to SA’s paltry first-innings total of 128, WA were dismissed for 299 late on day two, giving them a sizeable first-innings lead of 171.Green and Cameron Bancroft combined for a 101-run stand on a green deck to guide WA out of early trouble, before Hilton Cartwright and Josh Philippe powered home the advantage.The Redbacks lost pace bowler Brendan Doggett to a suspected side strain early in the day, but Nathan McAndrew picked up the slack to finish with 5 for 84, ripping through WA’s tail. David Grant was the pick of the bowlers with 4 for 76.South Australia made a strong start to their second innings through Jake Weatherald and Henry Hunt by reaching 0 for 68 at stumps, reducing WA’s lead to 103.Bancroft was the first wicket to fall on Thursday when he edged Grant behind. Green looked ominous during a knock that featured seven fours and a six. But he fell shortly after Bancroft’s dismissal when he skied an attempted pull shot off Grant.Green started the domestic season with a classy 106 against South Australia. But his next five knocks in Shield and one-day cricket returned scores of 2, 5, 7, 13 and 9 in what marked a rare form slump for the 22-year-old.His half-century on Thursday was welcome news for both WA and Australia’s national selectors, who view Green as a vital part of the Ashes puzzle.At 5 for 250, WA looked set to tear the game wide open. But Cartwright’s dismissal lbw to Grant sparked a collapse of 5 for 49 to give the Redbacks some hope.