But the start of the year was not easy, said Halep, who looked below-par in losing to lower-ranked Aryna Sabalenka in the quarter-finals in Adelaide the week before Melbourne.
“I got a little bit down with my head after that loss. I felt tired when I came here, I don’t know why,” said Halep, one of the most popular figures in women’s tennis.
“Maybe because I stress myself too much, and I just lost the energy.
“But then after the first match (in Melbourne) I got the energy back and I felt like my game is going at the higher level. I feel good.”
Halep is at a loss as to why she felt sluggish.
“I did off-season in Dubai, I worked hard. Then I had a few days off home,” she said.
“I don’t really know exactly what happened. Maybe mentally I was a little bit stressed, and I lost energy.
“As I said, day by day and when I had to start the tournament, I was down.
“But now it’s good and I don’t want to think about that anymore.”
After top seed Ashleigh Barty, Halep — last year’s Wimbledon champion and a former French Open winner — is the highest remaining seed left in the draw.
Second seed Karolina Pliskova, holder and third seed Naomi Osaka, and history-chasing Serena Williams were among six top-10 seeds who lost in the third round.
Halep had defeated Mertens in two of their three previous meetings, including in the last 16 of the French Open in 2018, when she went on to win the title.
But Mertens prevailed the last time they met when she came from a set down in the final in Doha last year.
Halep drew first blood at a sunny Rod Laver Arena when she broke Mertens at the fourth break-point opportunity and consolidated for a 3-1 lead.
The 24-year-old Mertens, whose best performance at a Grand Slam came in reaching the Australian Open semi-finals in 2018, broke back and then stubbornly held serve for 4-3.
But Halep broke again for 5-4 and converted the fourth set point when Mertens put a forehand into the net.
The ninth game of the second set proved pivotal, as Halep finally got the break at the fifth opportunity before serving out the match.