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The 31 year-old won 6-4 7-5 in one hour and 29 minutes to set up a last-eight clash with American compatriot Jessica Pegula.

Keys required just one break of serve to take the first, converting the only break point of the set in the seventh game to put Muchova under pressure.

Both players held serve in the first eight games of the second, but three consecutive breaks, two of which went in favour of Keys, allowed the American to clinch victory against the seventh seed.

Elina Svitolina joined Keys in the quarter-finals after a routine victory over Eva Lys in the German capital.

Svitolina won 6-3 6-2 in just 71 minutes and will face either Alexandra Eala or Elena Rybakina in the next round.

Linda Noskova also booked her place in the last eight, defeating Diane Parry 6-2 6-2 earlier on Thursday.