Then followed the grind, ending with Williams forcing one final exhausted forehand out of the Romanian.
"She's beaten me on an occasion or two or three", Williams said of her next opponent. "It's really cool to be back out and playing again on this court", Williams said in her courtside interview.
Williams prevailed 6-1 4-6 6-4 in a pulsating battle on Rod Laver Arena, keeping alive her hopes of winning the tournament for a third time in four years.
Until facing Halep, Williams had dropped a total of only nine games through three matches in the tournament.
Blown away in the first set before battling back impressively to force a decider, Halep remained upbeat despite slipping to a ninth defeat to Williams in their past 10 meetings.
She also described her 2017 Australian Open win, saying, "I think the last time I was here, I was actually pregnant and playing at the same time, which is insane".
Following Williams' triumphant win, her entire team, including sister Venus and coach Patrick Mouratoglou, burst into wild celebrations. I feel like it would make her a bit weaker if, at any point, she would feel like she needs me. "There's definitely something that's innate". I work so hard for every point'.
Halep broke Williams' serve in the first game and that was the only one she managed for the rest of the opening set.
"I am just a fighter, you know I just never give up".
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There's an opportunity to put them nine points behind us, but in a personal way it's going to be a special moment". But in these 14 games I started maybe only half of them, so it's a bit hard to be consistent in this case.
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Czech seventh seed Pliskova advanced earlier on Monday with a 6-3, 6-1 thrashing victory over two-time major champion from Spain, Garbine Muguruza.
The ratio of winners to unforced errors paints a picture of how Serena dug deep while raising her game: she committed 31 unforced errors overall to Halep's 12 but dominated the winners-count 44 to 24. "Game by game, I felt that my level is growing, and I had more confidence to go through it".
"I had to put my level up, I'm glad I handled the pressure", said Svitolina, who scored her biggest career win at the WTA Finals in Singapore last October.
" She is the No 1 player in the world and there is a reason why, she elevates her game". It was all one-way from then on. That was a good idea for her, I don't think it's a good idea for everyone, but for her it's the right choice.
Before the match, Halep said if she wanted to beat Williams at her best, she needed to move her around the court and dictate the pace of the game.
Svitolina is trying to emulate what Caroline Wozniacki did here previous year - coming to the season-opening major as victor of the season-ending WTA Finals and winning a breakthrough major title.
To get another shot at the prize, the remarkable American has to beat Karolina Pliskova in the quarter-final on Wednesday, and either Naomi Osaka - who famously ignored her meltdown in the final at Flushing Meadows last September to win the US Open - or the sixth seed, Elina Svitolina.
She made two more Grand Slam finals a year ago only to lose the finals at Wimbledon and the U.S. Open but this triumph over the world's leading player felt like another hugely significant step on the road to regaining her old dominance.