Serena Williams has kicked off her latest quest for an elusive 24th grand slam singles title with a straight-sets win over compatriot Kristie Ahn in the first round of the US Open.

Williams needed just under one-and-a-half hours to come through 7-5 6-3 despite falling a break down to her world No.96 opponent in both sets.

Four straight games turned the second set on its head and enabled Williams to belatedly find the momentum she required to claim victory.

Showing early signs of nerves, the former world No.1 then found another gear to fire off 13 aces and 28 winners.

A final serve forced Ahn into an error, handing Williams her 102nd US Open match victory, as she scored her first straight-sets win since the sport was put on a coronavirus hiatus.

“I was really happy with how I just fought for every point,” Williams said in an on-court interview after the match.

She had to regain her focus, she added, and “just be Serena and close it out.”

But her 40-year-old sister Venus crashed out, beaten 6-3 7-5 by Karolina Muchova, after committing 34 unforced errors at Arthur Ashe Stadium.

It was the two-time champion’s first opening-round loss at the tournament.

Muchova broke Williams to start the match and deployed a wide variety of shotmaking including a pinpoint serve, a slice backhand and some well-timed trips to the net to frustrate the former world No.1.

Williams was sharper at the outset of the second set and served at 5-4 with the chance to extend the match to a third, but a backhand sailed on break point and Muchova levelled at 5-5.

The 24-year-old Czech held at love the next game and sealed the win when Williams issued her third double fault on match point.

Spain’s Garbine Muguruza, looking determined to overcome her US Open jinx, beat Japan’s Nao Hibino to collect a snappy 6-4 6-4 first round win.

Flushing Meadows has not been a happy hunting ground for the former French Open and Wimbledon champion, who this year nearly added to her collection of grand slam trophies by reaching the final in Australia.

In stark contrast to her success at the other slams, Muguruza has struggled to make an impact on the New York hardcourts.

She has made it past the second round just once in seven previous visits to Flushing Meadows.

“I am happy with my performance I was expecting a tough match and it was,” said Muguruza.

Australian Open champion Sofia Kenin produced a dominant display to dispatch Belgian Yanina Wickmayer 6-2 6-2.

American Kenin has lifted two titles in 2020, including her maiden grand slam triumph in Melbourne.

No.7 seed Madison Keys needed just 55 minutes to beat Timea Babos 6-1 6-1 as the 2017 finalist looks to claim her elusive first grand slam title.

Sloane Stephens delivered an efficient 6-3 6-3 win over Mihaela Buzarnescu.

The 2017 champion, seeded 26th this year, smacked a backhand winner on match point to see off the Romanian after an hour and 16 minutes.