2018 Mutua Madrid Open – Women's Singles  Simona Halep was the two-time defending champion, but lost to Karolína Plíšková in the quarterfinals. Petra Kvitová won the title, also her third in Madrid, by defeating Kiki Bertens in the final, 7–6(8–6), 4–6, 6–3. Halep and Caroline Wozniacki were in contention for the WTA no. 1 singles ranking at the beginning of the tournament. Halep retained the top ranking when Wozniacki lost to Bertens in the third round.