And even though her opponent is ranked 16th entering the big-name match, Halep told USA Today that she can't even compare herself with Williams, a 23-time Grand Slam victor and seven-time Aussie Open champ.
Luckily, the seven-times Australian Open victor was on hand to console the 18-year-old as she became tearful after the match, telling her: "You did wonderful, you're so young...don't cry!"
The 37-year-old American, playing just the eighth tournament in the 10 months since her return from maternity break, has dropped only nine games in three matches so far in the first Slam of the year.
Her latest victim Yastremska was not even born when the American won her first major at Flushing Meadows in 1999.
Japan's No.4 seed Naomi Osaka, who beat Williams in the controversial final of last year's US Open, had a much tougher time in the third round at Melbourne Park on Saturday before finding a way past Hsieh Su-Wei from Taiwan 5-7 6-4 6-1.
She also shared a touching Instagram story of the moment she was hugged by Williams, with the caption: "I took huge experience, next time, I'm gonna use it!" "I liked that. It showed that she wasn't just there to play the match, she was there to win. That really broke my heart", Williams said. "She played unbelievable tennis and hopefully we'll see many more of her talents back in Melbourne".
Serena Williams is known for keeping a tenacious grip over the headlines as whatever she does set the tongues blabbering, always!
One is the top ranked player in the world and the other, seeded 16th, is arguably the best player in the sport. I think either way, regardless, I'll be ready for either opponent.
"When I was young I played against so many people like Lindsay Davenport, Martina Hingis and Monica Seles. everyone I faced was in the hall of fame".
Given all her time at No. 1 herself and time away from the game after giving birth, it has been a while since Williams took on a top-ranked player - in Doha in 2013 to be exact.
The powerful serve of Venus landed five aces to two for Halep who committed same number of double faults compared to four by Venus.
"I remember one time in particular against Venus at Wimbledon, as I was walking to the net, I started bawling", she said. "Young girls, young women, just want to go out there and do their best and want to win".
Williams admits that her sister "still intimidates me".
Halep admitted that she is not overawed by her opponent anymore, despite the lopsided head-to-head. Both women have legitimate claims to being the top player in the game.
John McEnroe and Chrissie Evert share their thoughts on Serena Williams' easy victory over Dayana Yastremska.
"But for sure, [Serena's] the best player in the world because she's won so many Grand Slams".