Riske will next face Serena Williams in Tuesday's quarterfinals.
Gauff will face Romanian Simona Halep now ranked no. 7 in the world in Round of 16 later on Monday.
This American - a qualifier, remember - is why Wimbledon 2019 will live long in the memory for all who have experienced it.
Gauff thrilled the crowds at Wimbledon throughout the first week and her efforts were noticed around the world, with the likes of Magic Johnson, the Miami Heat and Reese Witherspoon all offering their congratulations and best wishes.
That match on Friday cemented her place in history, as she fought back from two match points, including a second set tiebreak, to come out on top.
"It was really surprising because you don't really expect this kind of support when you're in another country, not your home country", a tearful Cori said in the wake of her defeat to Simona Halep. I learned what it was like to be under pressure. I'm just really happy and happy that people believe in me.
"I learned a lot". Coco is a nice girl, works really hard.
"Some of the stars are getting older and you get new stars". She will soon go on a mini holiday with her family.
"I'm happy that I could play my best tennis I'm not comparing myself with a year ago. I think she's obviously got a wonderful future ahead of herself".
How did she do it?
Coco Gauff's unexpected but remarkable run at Wimbledon is over. A lot can change in a year.
Coco became the youngest player at the Championships in the Open era when she qualified at Roehampton.
The 15-year-old lost to former world No. 1 Simona Halep in the fourth round. Muchova claimed six points more than Pliskova, earning six breaks from 13 opportunities and defending seven out of 12 break points she faced to prevail after a marathon.
Polona Hercog was the next to try to take her down, and it looked as though that was exactly what was going to happen when she won the first set. Gauff saved two match points when serving at 5-2 but Halep clinched the win when the teenager sent a forehand wide in the next game.
According to the Associated Press: Halep broke Gauff five times and took advantage of 29 unforced errors.
"It just shows that even though I'm young, my game gives these players a little bit of a hustle", Gauff said. "I had to take it to Ash".
Gauff's comeback spirit has been witnessed throughout the tournament and she got back on level pegging at 2-2, after the doctor and trainer paid a visit. Perhaps they are sensing something special for years to come, even though she was ranked No. 313 in the world entering the famed tournament in London.