Sonmez, who has enjoyed a strong 2026 so far, reaching the Australian third round and becoming the first Turkish woman in the Open Era to win a match there, is currently sitting at a career-high ranking of 67, and showed her vast improvement against the Spaniard.
There were seven breaks of serve in a topsy-turvy first set, before Sonmez saved three set points at 4-5, 0-40 to eventually take the opener. She had a far more straightforward time in the second set, sealing the win in an hour and 53 minutes.
Alexandra Eala also booked her spot in the second round, when she claimed a comfortable 6-3, 6-3 victory over former French Open finalist Anastasia Pavlyuchenkova.
It is the third year in a row that Eala has won her opening match in Madrid, but the Filipino has never made it past round two. 19th seed Elise Mertens now awaits.
Meanwhile, Elena Gabriela Ruse, who reached the first WTA 500 semi-final of her career just under two weeks ago in Linz, had to dig deep to see off Antonia Ruzic, 6-3, 4-6, 6-0.
However, there was no such luck for Eva Lys, who fell to a 6-4, 6-3 loss to Shuai Zhang. The German has struggled to find her best form after picking up an injury at the Australian Open, with Zhang ending a six-match losing streak.
Daria Kasatkina's tough 2026 continued with a heartbreaking 6-3, 3-6, 7-6(13) defeat at the hands of Daria Snigur. Kasatkina squandered four match points before succumbing in the 28-point final set tiebreak.
Over on the men's side of the tournament, 2014 US Open champion Marin Cilic secured a fine win over Zizou Bergs, coming from a set down to triumph 4-6, 6-3, 6-4.
The 37-year-old will now face Joao Fonseca in the second round - a player 18 years his junior.
2021 runner-up, Matteo Berrettini, was handed a 6-3, 6-4 loss by 20-year-old Dino Prizmic, and will now drop out of the ATP top 100 as a result.
Other winners on Wednesday included Hubert Hurkacz, Karolina Pliskova, Yulia Putintseva, Katie Boulter, Sofia Kenin and Katerina Siniakova.