Canada's Eugenie Bouchard moved on to the third round of the Qatar Open after another tight victory on Tuesday.
The 21-year-old defeated Denisa Allertova, of the Czech Republic, 7-6 (0), 7-5 in a battle of service breaks.
Bouchard will face China's Zheng Saisai following her 7-5, 6-1 upset win over Australian Open champion Angelique Kerber.
Kerber beat Serena Williams at Melbourne in the opening Grand Slam of the year on Jan. 30. The top-seeded Kerber split two matches in Germany's loss to Switzerland in the Fed Cup and received a first-round bye at Doha.
The tournament also lost its second-seeded player when Simona Halep, the 2014 champion, was beaten by Russian qualifier Elena Vesnina 6-7 (1), 6-4, 6-1.
Third-seeded Agnieszka Radwanska advanced after beating Ukraine's Kateryna Bondarenko 6-4, 6-4 while No. 4-seeded Garbine Muguruza beat Japan's Nao Hibino 6-2, 6-0.
Two-time Wimbledon champion and fifth-seeded Petra Kvitova defeated Barbora Strycova 7-6 (2), 6-4 and former No. 1 Caroline Wozniacki beat Australia's Daria Gavrilova 6-3, 6-3.
Zheng, ranked 73rd, made just eight unforced errors compared to 38 by Kerber and needed only 28 minutes to win the second set.
"The feeling is amazing," Zheng said. "For sure it wasn't her [Kerber's] best tennis today, but I'm happy I won that match."
Kerber, who withdrew from last week's Dubai Open with a right thigh injury, said: "I was not feeling good from the beginning. I think you have days like this sometimes. I don't know how many mistakes I did today. This is not my game ... that's all I can say."