Whoever reaches the final from that side of the draw will face either Austrian seventh seed Dominic Thiem or unseeded Italian Marco Cecchinato in Sunday's final.
Nadal returned with his famed forehand finally firing on all cylinders, earning back-to-back breaks to move within a hold of the set.
Nadal - who had his left wrist taped after losing his first set in his favourite tournament since 2015 - was 30-15 up and serving to make it a set apiece when the heavens opened again and no further play was possible.
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The days between games couldn't come at a better time for Mr. 82 Games (for the first time in the regular season in his career). LeBron James kept Cleveland close despite Golden State's torrid shooting with 10 points, five assists and four rebounds.
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Those penalties will remain in place "until the United States government eliminates tariffs imposed", Mexico's government said . The same was true of Mexico and Canada, with the latter's leader lashing out at Trump in recent days over the matter.
As he took his position to serve for his third set point, he turned around and saw a spectator struggling in the stands and immediately moved to the support staff and asked them to get the person some medical assistance.
Nadal, who was broken five times by Schwartzman in a stuttering start to the match, appeared to be relishing the warmer conditions in Paris and his greater weight during the baseline exchanges was proving pivotal.
Now, with her talent dazzling but her head not dazzled, and without having lost a set here, and while having made the 31-year-old Sharapova look briefly like something of a relic from a long-gone era, Muguruza looks like the most polished force in a shiny set of women's semifinals set for Thursday, pending cloud permission.
Schwartzman had a point to lead 4-2 but a forehand error allowed Nadal off the hook.
Play will resume on Thursday, with both matches finely poised.