No.3 seed and defending champion Naomi Osaka moved into the third round of the US Open after qualifier Olga Danilovic withdrew ahead of their second-round clash due to a medical reason. Danilovic posted on social media that she has been dealing with a viral illness this week.

Osaka, the winner in 2018 and 2020, has now reached the third-round stage in Flushing Meadows in all six of her appearances. The Japanese player defeated Marie Bouzkova in straight sets in her opener.

"I am so sad to have to [withdraw] from my match this morning," wrote World No.145 Danilovic on social media. "I have been feeling unwell these past few days dealing with a non-Covid related viral illness. I was really looking forward to playing against Naomi on Arthur Ashe Stadium today, but not to be this time."

Danilovic, 20, was competing in her first US Open main draw. The Serb is a former junior No.5 who won the 2018 Moscow River Cup as a lucky loser in her WTA main draw debut. She owns three Top 20 wins in as many encounters with opposition in that range, and this July made back-to-back WTA quarterfinals in Budapest and Palermo.

Halep makes use of roof boost to ease past Kucova

Another significant beneficiary of Danilovic's withdrawal was No.12 seed Simona Halep. Originally scheduled second on Court 17 with rain forecast all day, the 2015 semifinalist was moved to Arthur Ashe Stadium in Osaka's stead - enabling her to get her second round done and dusted even as the heavens opened.

Halep's 6-3, 6-1 defeat of lucky loser Kristina Kucova in just 68 minutes moved the Romanian into the third round here for the first time since 2016. She maintained her recent serve improvements, winning 76% of the points behind her first delivery; and, once she began pummelling her forehand from line to line in earnest, reeled off nine of the last 10 games of the match.

Turning point: Kucova had lost all four of her previous Grand Slam second-round appearances, but the 31-year-old Slovak has enjoyed an uptick in recent form. In March, she held match point over eventual champion Ashleigh Barty in Miami. In July, she reached her first WTA final in Gdynia - winning the longest match of the year so far, and sixth-longest of the Open Era, in the quarterfinals over Ekaterine Gorgodze.

After falling down a break after a slow start, Kucova took advantage of a brace of Halep double faults to smack a return down the line to level at 3-3. Clubbing her double-fisted groundstrokes with renewed vigour, the World No.111 advanced to 30-0 in the next game and seemed to have seized some momentum.

Halep put a stop to that with alacrity. The former World No.1 pulled off a swashbuckling off forehand winner, and thereafter dominated every facet of the game. In the blink of an eye, Halep turned a sticky situation into a procession.

"I had to start to do exercises like when I was kid, slowly and just walking on the toes. It was tough recovery, like four, five hours a day."

- Simona Halep on overcoming the calf tear that sidelined her for three months this year.

Quotable: After the match, Halep opened up about the physical and mental challenges of dealing with the calf tear that sidelined her between May and August, and ruled her out of Roland Garros, Wimbledon and the Olympic Games.

"At the beginning was very tough," she said. "Because never happened to me, this kind of injury, to have a muscle broken in two spots, big spots. So it was challenging, because I never faced something like this before. I didn't know how to handle the pain. And also, the recovery, I had to start to do exercises like when I was kid, slowly and just walking on the toes. It was tough recovery, like four, five hours a day.

"It's been a little bit more difficult, the mental part after the injury. After I got healthy [and came] back, I could play without pain, [but] I was still scared of it. Mentally I was a little bit down. Then I came to Cincinnati, and I did another thing, similar to what I had at the calf, but much smaller.

"But, you know, I have a lot of experience now, many years with pain... like every match, probably, but not that big injury. And now I feel happy that I feel good, no pain, no struggling anymore with the mental part. So I'm just happy to be on court and trying my best to win every match I play."

What's next for Halep: A third-round clash with a power hitter: either No.19 seed Elena Rybakina or former World No.4 Caroline Garcia awaits. Halep dominates the head-to-head against Garcia, leading 7-1, but needed a third-set tiebreak to quell Rybakina in their only previous completed match, in the 2020 Dubai final.

Rhea Nall/USTA

Jabeur in a hurry to dismiss Osorio Serrano

No.20 seed Ons Jabeur didn't risk getting caught up in the forecast rain, turning in a remarkably efficient performance to swat aside Maria Camila Osorio Serrano 6-0, 6-1 in just 53 minutes.

The Tunisian struck 16 winners and won eight out of eight net points to dominate her opponent. The result puts Jabeur into the third round of the US Open for the third year in a row, and extends her streak of reaching at least that stage to her past eight majors.

Two double faults put 2019 junior champion Osorio Serrano on the back foot immediately, but the 19-year-old World No.80 was generally unable to find her game amidst the magic coming off Jabeur's racquet. Among the hot shots were a leaping backhand down the line, hit while scrambling diagonally backwards; a reflex forehand winner off a net cord that had unexpectedly bounced the ball behind Jabeur; and a remarkable flat forehand crosscourt after some spectacular defence.

Jabeur will next face No.15 seed Elise Mertens for the first time after the Belgian dispatched qualifier Valentini Grammatikopoulou 6-2, 6-2. Mertens did not quite dodge the showers as efficiently as Jabeur: she was a game away from victory when the rain arrived, but after a short delay returned to wrap that game up on her second match point.