In Keys, she'll face someone who is in the quarters for the fifth time in the last seven majors, including a runner-up finish at the 2017 U.S. Open and a semifinal showing in Paris a year ago. No. 10 Karen Khachanov of Russian Federation.
Novak Djokovic continued to ease through the French Open draw with a dominant victory over Jan-Lennard Struff in the fourth round. The American had found her range, though, and she went 3-0 up in the second, pulling her opponent around the court - just as she did in beating 23-time Grand Slam champion Serena Williams in the previous round.
The French Open is the only Grand Slam without a retractable roof in case of rain.
Halep now faces an an even younger player in 17-year-old American Amanda Anisimova, who defeated Spanish qualifier Aliona Bolsova 6-3 6-0 on Monday.
The men's semifinal matches will be on Court Philippe Chatrier, the largest court on the grounds.
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She is the first player born in the 2000s to make the last eight at a slam.
However a quite sensational rally was won by Anisimova as she eventually held to retain her three-game lead.
Anisimova will play her first major semifinal against either 14th-seeded Madison Keys or eighth-seeded Ashleigh Barty.
Play is likely to resume at midday local time on Thursday, with two of the women's quarter-finals.
The decision left world number one Djokovic and defending women's champion Halep facing having to play their quarter-finals tomorrow and semi-finals on Friday if they get through.
The victor of that match will face either last year's runner-up Dominic Thiem or big-hitting Russian Karen Khachanov, who knocked out Juan Martin del Potro in the fourth round.
Djokovic already accomplished the feat in 2015-16.