Garbine Muguruza had to battle to beat defending champion Aryna Sabalenka in Doha, winning a second round thriller 6-2, 6-7(5), 6-3.
Muguruza took the first set with relative ease after setting up a 4-0 lead, but the second proved more difficult.
The Spaniard served for the match at 6-5, but allowed Sabalenka back in to level the tie.
In the decider Muguruza came from a break down and then won the last four games to clinch victory.
“Today I was facing a top player, and I knew I have to go out there with a big, fighting spirit, and strong to be able to win against these tough opponents,” Muguruza said.
In other matches, No. 2 seed Karolina Pliskova needed three sets to finish off Tunisian Ons Jabeur 6-4, 4-6, 7-5. Pliskova will meet Jessica Pegula, who beat Jelena Ostapenka, in the next round.
Muguruza will face Maria Sakkari in the next round after Sakkari demolished Madison Keys 6-2, 6-2.
Petra Kvitova breezed to a 6-1, 6-3 win over Russian Anastasia Pavlyuchenkova, and Anett Kontaveit was too good for Angelique Kerber, winning 6-1, 6-4.
Kvitova and Kontaveit will meet in the quarterfinals.
Top-seeded Elina Svitolina hit 26 winners for a comfortable 6-1, 6-2 win over Misaki Doi of Japan.
Victoria Azarenka of Belarus notched a 6-4, 6-2 win over Laura Siegemund of Germany.