Muguruza and Halep through

Garros, leaving the men’s quarterfinals suspended in progress. That might very well have been a relief to 10-time champion Rafael Nadal, who was not at his best and dropped a French Open set for the first time since 2015.

Nadal lost the opener 6-4 against 11th-seeded Diego Schwartzman but began to play better after a rain delay of just under an hour.

Nadal was serving for the second set at 5-3, 30-15, when another shower came and action was halted for the day.

In the other men’s quarterfinal, No 3 Marin Cilic and No 5 Juan Martin del Potro were at 5-5 in a firstset tiebreaker when the matchup of past US Open champions was stopped.

In addition to Halep versus Muguruza, the other women’s semifinal is the first at the French Open featuring two Americans in 16 years: No 10 Sloane Stephens against No 13 Madison Keys in a rematch of last year’s US Open final. Stephens won that one, as well as the only other match the long-time friends, Fed Cup and Olympic team-mates have played against each other.

Muguruza had lost all three previous times she faced Sharapova, whose five grand slam titles include two at Roland Garros.

But they hadn’t played since 2014, back before Muguruza was as confident and successful as she is these days.

Muguruza has not dropped a set in this year’s tournament and dominated Sharapova with booming groundstrokes and superb returning that earned six breaks.

‘‘When you’re facing somebody that also has an aggressive style of game, I think it’s about who takes the command, who takes the first opportunity,’’ Muguruza said about facing Sharapova.

Sharapova certainly didn’t do that. She double-faulted three times in the opening game, part of her 27 unforced errors.

It took her 29 minutes just to gain a game, and by then she already trailed 4-0.

‘‘She did a lot of things better than I did,’’ Sharapova said.

Sharapova didn’t play a point in the fourth round, because Serena Williams pulled out of their muchanticipated showdown with an injured chest muscle. Muguruza essentially got that round off, too: Her opponent, Lesia Tsurenko, quit with an injury after only two games.