The top two seeds Maria Sharapova and Yanina Wickmayer were both made to battle their way into the quarter-finals of the Auckland Classic on Wednesday while other seeds crashed around them.
Third seed Svetlana Kuznetsova and the seventh to ninth seeds Elena Vesnina, Sofia Arvidsson and Carla Suarez Navarro all went out in the second round following sixth seed Kimiko Date-Krumm who lost her first match.
Sharapova had been expected to dispatch Renata Voracova with ease but dropped her serve early to trail 1-3 before finding her rhythm to take out the first set 6-3.
A series of double faults in the second set saw her drop serve twice to be down 3-4, however her experience showed as she rallied again to take the set and the match 7-5.
"I'll definitely have to improve for the next one," said Sharapova who will play Greta Arn in the quarter-finals.
Arn dropped the first set in her second round match against Arvidsson but fought back for a 4-6, 7-5, 7-5 victory.
Wickmayer, the tournament defending champion, was also stretched to three sets to dispose of Sabine Lisicki 6-2, 3-6, 6-2.
At one-set all, Wickmayer said she knew she had to lift her game and after securing a service break early in the decider she was not troubled again.
"After the second set I knew I had to do something more to win the match otherwise I knew it was going to go the other way," Wickmayer said.
"I tried to be a lot more aggressive and lift my game, it wasn't that easy with the wind and maybe a risk in making more mistakes, but that's the game I wanted to play and I'm so happy now."
In the quarter-finals Wickmayer will play Romanian Simona Halep who beat Russian Vesnina.
Halep was leading 2-6, 6-4, 4-0 when Vesnina retired complaining of dizziness.
Third seed Kuznetsova, a two-time Grand Slam winner, blamed her inability to put away the big points for her loss to China's Peng Shuai in a three-set marathon that lasted more than two hours.
Kuznetsova won the first set 6-2, but Peng took the next two 6-4, 6-4.
"I was leading all the time, but when you don't take chances, the opponent will take them," Kuznetsova said.
"I felt like I was moving well, but at moments I didn't choose the right shot."