Serena Williams moved into the third round of the US Open and a step closer to the magic 24.

The 38-year-old American battled past Russian world number 117 Margarita Gasparyan 6-2 6-4 on Arthur Ashe.

Williams requires one more grand slam to equal Margaret Court's singles record, but has been stuck on 23 titles since winning the 2017 Australian Open.

But with the field decimated by withdrawals coupled with the shock defeat suffered by top seed Karolina Pliskova on Wednesday, Williams has an increasingly clear run to a seventh Flushing Meadows crown.

Williams will meet fellow American and former champion Sloane Stephens, who beat Olga Govortsova 6-2 6-2, in round three.

"I'm just happy to get through and try to focus on the next round," said Williams.

"Sloane is such a good player, she plays so smooth. She looks like she is not taking a lot of energy and then suddenly bam! Five winners."

Tsvetana Pironkova stunned Garbine Muguruza to reach the third round of her first tournament for more than three years.

The Bulgarian, a former Wimbledon semi-finalist, retired following the Championships in 2017 and gave birth to her first child, son Alexander, but decided to give the tour another shot earlier this year.

The suspension of professional sport delayed her comeback but she has certainly made up for lost time.

Having beaten Liudmila Samsonova in the first round, Pironkova showed her prowess on fast courts has not diminished by knocking out 10th seed Muguruza 7-5 6-3.

The 32-year-old said: "I guess I was mentally prepared. It's not something super new. At the same time, I was feeling well, I was moving well, I was hitting the ball well. Why not win?"

Muguruza made a late decision to come to New York and knew her lack of preparation could cost her.

That did not stop her showing her frustration with a spectacular smash of her racket after losing the first set, snapping the head clean off.

Australian Open champion Sofia Kenin continued her winning streak at grand slams with a 6-4 6-3 victory over 17-year-old Canadian Leylah Fernandez.

There was an all-teenage battle on Louis Armstrong Stadium, where 19-year-old Amanda Anisimova defeated 16-year-old Katrina Scott 4-6 6-4 6-1.

Anisimova reached the semi-finals of the French Open last summer but pulled out of the US Open following the sudden death of her father and coach Konstantin.

She blew a kiss to the sky after completing the comeback and said she was inspired by memories of her father.

"I was just thinking about him the whole time and it was giving me a lot of energy to pump me up and get myself going," she said.

Victoria Azarenka won the all-Belarusian clash with fifth seed Aryna Sabalenka, 6-1 6-3.