Muchova had 23 wins to Stephens’ 22, but the Czech made 32 unforced errors to the unseeded American’s 25.
Stephens lost the final at Roland Garros in 2018 after winning her only Grand Slam title at the US Open the previous year.
You will then face the former doubles champion at the French Open Barbora Krijcikova, who gave her best singles performance in a Grand Slam by reaching the fourth round.
Krejcikova used a heavy forehand to sink the fifth seed Elina Svitolina Ukraine 6-3, 6-2 at Stade Philippe Chatrier. She pushed both hands in the air and screamed with glee after winning her first match point.
Krizhikova also reached the fourth round at Roland Garros last year.
Unseeded Krejcikova’s best single performance in any other discipline was to reach the second round at the Australian Open.
She is a former number one doubles player with titles from the French Open and Wimbledon in 2018.
Information from The Associated Press was used in this report