The second seed came into Saturday's clash at the WTA International hard-court event aiming to reach her seventh Tour final, but China's new number one was forced to pull out with a lower back injury in the first set.
Peng was 3-1 down at the time, after Bencic had broken her in the opening game of the match.
The 17-year-old Bencic now progresses to her first WTA final, and should she win, she will become the youngest victor on Tour since Tamira Paszek of Austria, who was 15 when she won the Slovenia Open in 2006.
Like Bencic, Riske has reached her first Tour final, after she overcame another home hope, Zheng Saisai.
Sixth seed Riske was utterly dominant throughout the encounter, dispatching her opponent 6-0 6-1 in just over an hour.
The American was ruthless on her own first serve, winning 86 per cent of the points and she capitalised on her openings, winning nine out of 11 points on Zheng's second serve and breaking on five occasions.
Zheng did not get on the board until game four of the second set, but by then it was far too late as Riske eased into a final showdown with Bencic.