This was Keys’ first win over the Romanian in three career meetings.
This is the American’s 12th quarter-final at the WTA 1000 Series level. She was also a semi-finalist in Madrid earlier this month, where she lost to world No 1 Iga Swiatek.
Keys, ranked No 16, could run into Swiatek, the top seed, again if the Pole can get past German Angelique Kerber in her fourth round match later on Monday.
Ahead of her win today, the American beat Colombian Camila Osorio (6-0, 4-6, 6-3) and Brazilian Beatriz Haddad Maia, the No 12 seed (6-4, 4-6, 6-3).
Cirstea, ranked No 32, won against qualifier Brenda Fruhvirtova (7-5, 6-0) and No 6 seed Marketa Vondrousova (7-6 (1), 6-3) earlier in the tournament.
Rome WTA 1000, other round of 16 results (Foro Italico, clay, USD 5.509.771):
- Qinwen Zheng (7) beat Naomi Osaka: 6-2, 6-4
- Elina Svitolina vs. Aryna Sabalenka
- Maria Sakkari vs. Victoria Azarenka
- Danielle Collins vs. Irina-Camelia Begu
- Iga Swiatek vs. Angelique Kerber
- Rebecca Sramkova vs. Jelena Ostapenko
- Coco Gauff vs. Paula Badosa
