Dubai: Holder and top seed Elina Svitolina toiled for more than two hours before finally overcoming Wang Qiang in her opening match at the Dubai Championships on Wednesday.
The Ukrainian worked hard to push past stubborn resistance from China's Wang, a lucky loser from qualifying, winning 6-1, 5-7, 6-2 to move into the quarterfinals.
Svitolina, this season's Brisbane champion, scraped through after saving nine of 11 break points against the world number 46 who entered the field after the withdrawal of Petra Kvitova.
"I was a little bit angry at myself that I didn't bring my best game in the end of the second set. I had to accept and bounce back," Svitolina said. "But I was very happy the way I was playing. In the end, a win is a win."
Former top-ranked players Karolina Pliskova and Angelique Kerber advanced to set up a battle of ex-number ones in the last eight.
Czech Pliskova, the third seed, beat Carla Suarez Navarro 5-7, 6-2, 6-4, while Germany's Kerber dispatched Italian qualifier Sara Errani 6-4, 6-2.
Sixth seed Kerber, who won the Sydney title in January before reaching the Australian Open semi-finals, leads the series with her Czech opponent 6-3, winning their last two matches.
"It was a tricky match, especially because she was playing well," Kerber said after taking the win over Errani on a secondary showcourt at the Aviation Club. "I don't care which court I play on. It's always important now to win the matches and focus more on my opponent, on my game, on the next round."
"I know how to play on the centre court. I know how to play on the outside court. It's just about your mentality, just focusing on not the schedule, more on the game."
Japan's Naomi Osaka, the next opponent for Svitolina, beat Anett Kontaveit of Estonia 6-2, 7-6 (7/5), while French fifth seed Caroline Garcia put out Ekaterina Makarova 6-4, 6-2.
Russian Elena Vesnina upset Roland Garros champion and fourth seed Jelena Ostapenko 6-1, 7-6 (8/6).
"It was a weird match. At the beginning, everything was going my way. I was playing very clean. Jelena was missing a lot of shots," Vesnina said.
"In the second set, she got back. She is a top 10 player, she is a Grand Slam champion. Of course, she will rebound, she will change something in her game. I had some kind of weird games on my serve. I will speak with my coach about that."
Pliskova, who headed the rankings from mid-July to mid-September, needed a fightback against Suarez Navarro after losing the opening set to her 27th-ranked opponent from the Canary Islands.
The Czech won the second set and survived a run of three consecutive breaks of serve in the third, taking a 4-3 lead over Suarez Navarro.
Pliskova reached 5-3 and served out the fighting win two games later in just over two hours.
"I was not playing the best tennis today, we were both a little bit struggling," Pliskova said after finishing with 11 aces, six double-faults and losing her serve five times.
"For me, this was the first match here after couple days of practice. I haven't had that many matches for last couple of weeks since I came from Melbourne.
"I'm just finding the rhythm of the matches. Finally, I got the win, so maybe it's better in the next match.
"These are the important wins - when you are not playing your best tennis."