Semien will start at shortstop and lead off Sunday against the Mariners, Greg Johns of MLB.com reports.
Semien has regularly been serving as the Athletics' table setter against left-handed pitching, but the leadoff assignment in the series finale is more notable with right-hander Felix Hernandez on the mound for Seattle. Nick Martini has been the Athletics' primary option atop the order of late versus righties, but since he was optioned to Triple-A Nashville a few days ago, it looks like those duties could now fall to Semien. Though he's fared better versus southpaws this season with a 112 wRC+, Semien has at least hit a palatable .263 in his matchups against right-handed pitching.