American Serena Williams, right, is congratulated by Romania’s Simona Halep after Williams won their fourth-round match at the Australian Open in Melbourne, Australia, today. (AP photo)

Oops.

Williams, wearing headphones, quickly pivoted and retreated back into the tunnel she’d just emerged from, making way for her fourth-round opponent Monday night. A couple of hours later, when Williams wrapped up her 6-1, 4-6, 6-4 victory over Halep in a contest filled with momentum swings and fantastic play by both, it was clear which woman really is No. 1, regardless of what the current rankings say.

“It was a really intense match, and it was some incredible points,” said Williams, who improved to 9-1 against Halep and now meets 2016 U.S. Open runner-up Karolina Pliskova in the quarterfinals. “I love playing tennis and I love to play here and I love to be back out here.”

Williams sat out last year’s Australian Open after giving birth to a daughter months earlier and dealing with health complications. Since returning to the tour, she has reached the past two Grand Slam finals, losing both — and leaving her still one shy of equaling Margaret Court’s record of 24 major singles titles.

American Serena Williams celebrates after defeating Romania's Simona Halep in their fourth-round match at the Australian Open in Melbourne, Australia, today. (AP photo)

Halep offered up the first real test Williams, 37, has faced at this year’s Australian Open, where she is bidding for an eighth trophy, pushing her throughout a back-and-forth matchup that was a fascinating mix of power, court coverage and shotmaking.

Pliskova advanced earlier Monday with a 6-3, 6-1 victory over two-time major champion Garbine Muguruza.

The other quarterfinal on that side of the women’s bracket features No. 4 Naomi Osaka against No. 6 Elina Svitolina. Osaka, who beat Williams in last year’s U.S. Open final, edged No. 13 Anastasija Sevastova 4-6, 6-3, 6-4 in the fourth round, while Svitolina got past 2017 U.S. Open finalist Madison Keys 6-2, 1-6, 6-1.