Medvedev topped Karen Khachanov in an all-Russian semi-final before Monfils announced he was no shape to face Nadal a couple of hours after injuring his left ankle in a quarter-final win over Roberto Bautista Agut.
Eighth seed Medvedev prevailed 6-1 7-6(6) over Khachanov in a victory that will make him Russia's top-ranked player on Monday but felt he should have finished his rival off earlier.
"I played amazing, but there were two moments in the match I should have won easier," the 23-year-old Medvedev told ATP Tennis Radio.
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"I was confident about myself and then there were some mistakes I should not have done. Finally, (I let Khachanov) back in the match."
Medvedev was upset at letting a break advantage slip away on two occasions in the second set, including at 5-4 as he tried to serve out the match.

Rafael Nadal

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He also was unhappy that after he led the tie-break 2-1, Khachanov scored four consecutive points.
But Medvedev, who has not lost a set in the tournament, fought back to tie at 4-4 and after two more ties won the final two points and the match.
Frenchman Monfils also needed to win a tie-break to defeat Bautista Agut in the third set of their rain-delayed quarter-final match.
Monfils won out 6-4 3-6 7-6(2) in a match that took two days to complete because of rain but the ankle injury was clearly hampering him and he withdrew from the tournament two hours later.

Rafael Nadal in Montréal 2019

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The ankle started bothering Monfils in the second set, but after a visit by a physio he elected to continue.
Bautista Agut won the set comfortably and the third went to a tie break which Monfils led 5-0 before sealing victory with a huge forehand winner after more than two hours.
Monfils said he was proud of the way he played on against Bautista Agut.
"I think I showed today I was a fighter," he said.
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