NEW YORK -- Aryna Sabalenka went to sleep on Sunday night not knowing if she would wake up to the news that she would be the new World No.1 after the US Open.

Sure, she could have stayed awake to watch the end of reigning World No.1 Iga Swiatek's showdown with Jelena Ostapenko

Or, she could focus on her current task of trying to sweep the hard-court majors this season. She chose the latter.

US Open 2023: Draw Order of play | Scores | 411

"I was really curious what's going on there, but I pushed myself so hard to don't watch the score, just go to bed," Sabalenka said. "I know if I would see the result yesterday, would be really tough to sleep.

"I was just like, No, I have a match tomorrow, I have to focus on that, it doesn't matter."

That discipline is precisely why Sabalenka found herself in a position to even contend for the No.1 ranking, which she will take over from Swiatek after the US Open. She played her first match as the presumptive No.1 on Monday night and showed no sign of any let down, defeating No.13 Daria Kasatkina 6-1, 6-3 to advance to Wednesday's quarterfinals.

A semifinalist last year, Sabalenka will face China's Zheng Qinwen to return to New York's final four. 

'She put a dream seed in my heart': Zheng Qinwen credits Li Na at US Open

Sabalenka played a laser-focused match to break Kasatkina seven times and fired 31 winners to 23 unforced errors. She limited Kasatkina to just 47 percent of her first-serve points won.

Asked about waking up to the news that Swiatek lost, Sabalenka said she was stunned.

"I mean, to be honest, I didn't want her to lose," Sabalenka said. "I was watching that match and I was just like, 'C'mon, win it'. After some of the upsets, I thought she's probably going to make it to the final. I was just thinking I'm going to do everything I can possible and impossible to reach the final and decide everything on court.

"So I was sad, but at the same time I have to be happy because it's not like she just fell. All year I've been playing well. I put her under pressure.

"I was just sad and happy at the same time. I didn't want to become World No.1 like this. I want a battle for this."

Changing of the guard: Sabalenka's Road to No.1 | Swiatek reflects on her 75-week run

Sabalenka may get her battle after the US Open when the two jockey for the prestigious mantle of year-end No.1. It's a challenge that Sabalenka embraces, one that she credits Swiatek for igniting and maintaining.

"I'm really happy that we have Iga on tour, someone who motivates me, especially me, to keep pushing, keep trying, keep improving myself," Sabalenka said. "I think that's the real sport, when you push yourself to the limits and you try to get better.

"I feel like it's great that we're playing great this year, we're pushing each other. I really hope we'll keep doing that and we'll keep facing each other in the very last stages of the tournaments, we'll battle against each other. This is what sport is."

"I'm focusing on US Open. I don't want to celebrate anything before the end of the US Open. I just want to focus on this tournament more than on World No. 1."

- Aryna Sabalenka

For now, Sabalenka must put aside her career achievement to focus on the task at hand. Along with Marketa Vondrousova and Ostapenko, Sabalenka is one of three major champions left in the draw. She is also the only remaining Top 5 seed, after Swiatek, Jessica Pegula, Elena Rybakina, and Ons Jabeur bowed out.

"I was No.2, now I'm No.1," Sabalenka said. "Of course, it's a great achievement. It's one of the goals. It's great to say that, yeah, I've been World No. 1. But it's not changing anything.

"You still have to bring your best tennis. Players have nothing to lose against you, they're playing their best tennis, so sometimes it's even tougher. Anybody can beat anybody. I'm trying to focus on myself and improve myself more, doing everything to bring my best tennis to each match."