Caroline Wozniacki swept aside Petra Martic in a 6-2 6-2 victory on Wednesday to reach the quarter-finals of the Auckland Open and continue her impressive run of form.
The Dane ended the 2017 season with the biggest title of her career at the BNP Paribas WTA Finals and has carried this form with her into the new year.
After taking 58 minutes to beat Madison Brengle in her first-round match on Tuesday, Wozniacki needed only 53 minutes to see off the unseeded Martic, despite dropping her serve in the first game of the second set.
Wozniacki served strongly to take early control, winning 89 per cent of points on first serve and putting her Czech opponent under pressure with deep groundstrokes.
"I'm trying to stay aggressive," Wozniacki said. "I still made some unforced errors. It's taking a few matches to play like 100 percent but I still felt I played pretty well today."
Wozniacki broke Martic's serve in the second and eighth games to take the first set in 24 minutes. She quickly recovered after dropping serve to open the second set, breaking back in the fourth and sixth games.
"I've worked on everything I need to work on in the off-season," Wozniacki said. "Now it's just a matter of working out how to incorporate that into your match or into a situation where you maybe get nervous."
Halep hit 21 winners and won five out of eight break points to prevail 3-6 6-1 6-2 and secure her No 1 status going into next week as well as the top seeding for Melbourne, ahead of Wozniacki and Garbine Muguruza.
However, the going was far from easy for the Romanian who was caught off guard by Duan’s big hitting early on, with the Chinese taking the lead after blazing through the first set.
It was not until her rival suffered a thigh problem that Halep snatched back control of the match, turning things around in the second set to break the world No 91 twice for a 5-1 lead before levelling the contest.
Duan took a medical timeout and returned for the decider with heavy strapping on her left thigh but Halep quickly raced to a 5-0 lead, eventually securing victory after 97 minutes.
"I knew that she is playing really hard and hitting every ball. I knew I had to be strong on my legs," Halep said.
"After the first set I was thinking I have to find rhythm, to stay closer to the baseline and open the court better, which I did... It was a very tough match."
Additional reporting by AP