Elise Mertens came from behind to clinch a semi-final spot at the Dubai Tennis Championships but Coco Gauff bowed out on Thursday.
Mertens said she "just didn't want to let go" after coming back from the brink to end Jessica Pegula's fine run with a 5-7 7-5 6-0 victory.
The Belgian looked to be heading out when she faced a 5-2 deficit in the second set, yet saved three match points to force a decider.
"I kept on fighting and that was the spirit today," said world number 18 Mertens.
Former world number one Muguruza is Mertens' semi-final opponent, the Spaniard having come from a set down to beat third seed Aryna Sabalenka 3-6 6-3 6-2.
Muguruza lost out to Petra Kvitova in the final of the Qatar Open last week, but looks on top form as she closes in on another championship match.
Sabalenka fell foul of Muguruza in Doha, and the Belarusian was unable to gain revenge as the ninth seed earned a 16th win of the season.
There was to be no place in the last four for 16-year-old Gauff, who came up short against world number 54 Jil Teichmann.
Gauff, who defeated Teichmann twice in Australia in February, was broken to go 5-3 down in the first set and could not respond.
A second break in Teichmann's favour put Gauff on the back foot in the second, and the Swiss then saved three break points to nose into a 3-1 lead.
The contest was settled on Gauff's serve – Teichmann taking the match at the second time of asking to tee up a last-four clash with Barbora Krejcikova, who beat Anastasia Potapova 6-0 6-2 in a little over an hour.
Krejcikova was beaten in the doubles final last year, and has claimed her place in the singles semi-finals of a WTA 1000 event for the first time.