For two sets, Riske absorbed every last punishing blow from the seven times champion, while her own lightweight jab inflicted some early pain and then levelled the see-saw encounter at 1-1.
Williams' blows, however, became too frequent and too heavy in the final games, with the weaponised serve pushing the world number 55 Riske back on her heels, while her return worked like an old slugger's right-hook.
Having knocked out world number one Ash Barty in the last round and having spent over nine hours on court en route to the quarters - the longest path to the last eight of anyone since 2011 - Riske had shown a streak of pure Pittsburgh steel.
But Williams, who is hunting down a record-equalling 24th Grand Slam singles title, is tennis tungsten, and recovered superbly from a brief wobble in which she lost the second set and fell a break behind in the third.
She eventually charged over the finish line after more than two hours on court, bringing up match point when Riske netted a forehand and completing the job with her 19th ace of the match to set up a clash with unseeded Czech Barbora Strycova, who beat the UK’s Johanna Konta in straight sets (7-6(5) 6-1).
Former world number one Simona Halep reached her second Wimbledon semi-final after overcoming an early onslaught to deliver another textbook example of matchplay tennis and dispatch China's unseeded Zhang Shuai 7-6(4) 6-1 on Tuesday.
Seventh-seed Halep now faces Ukrainian eighth seed Elina Svitolina, who beat unseeded Czech Karolina Muchova in straight sets (7-5 6-4), as she bids to reach the final for the first time having lost to Eugenie Bouchard in the last four in 2014.
