Rome - Simona Halep's hold on the No 1 ranking received a boost when American opponent Madison Keys withdrew from their third-round match at the Italian Open on Thursday due to a right rib injury.
Halep needed to reach the quarter-finals and progress further than No 2 Caroline Wozniacki to hold onto the top spot, and she has accomplished the first task.
Wozniacki was facing 15th-seeded Anastasija Sevastova in a late match on the red clay of the Foro Italico.
Keys also withdrew from the doubles competition, where she was partnered with Venus Williams.
Last year in Rome, Halep rolled her ankle in the final and lost a lead and the championship to Elina Svitolina.
Halep will next play either US Open champion Sloane Stephens or seventh-seeded Caroline Garcia.
Svitolina also reached the quarter-finals, overcoming a poor start in beating 14th-seeded Daria Kasatkina 0-6, 6-3, 6-2. She'll next face either former No 1 Angelique Kerber or Maria Sakkari of Greece.
This is the last major warmup for the French Open, which starts in 10 days.
Results from the ATP and WTA Masters tournament in Rome on Thursday (x denotes seeded player):
Simona Halep (ROM x1) bt Madison Keys (USA x13) retired
Jelena Ostapenko (LAT x5) bt Johanna Konta (GBR) 2-6, 6-3, 6-4
Elina Svitolina (UKR x4) bt Darya Kasatkina (RUS x14) 0-6, 6-3, 6-2