Former French Open champion Jelena Ostapenko is the latest player to withdraw from the U.S. Open, organisers of the tennis Grand Slam have confirmed.

The 23-year-old, who won her only major in 2017 at Roland Garros but has slipped to No 42 in the world rankings, last played on the WTA Tour in February at the Qatar Open where she reached the last 16.

Six of the world's top 10 women have already withdrawn from the U.S. Open, which begins next week and will be played in a 'bio-secure bubble' to combat the spread of the novel coronavirus.

Top-ranked Ash Barty, world number two Simona Halep, defending champion Bianca Andreescu, Elina Svitolina, Kiki Bertens and Belinda Bencic have all decided to skip the event in New York.

Andy Murray

Meanwhile, Andy Murray stepped up his return to competitive tennis with an impressive three-set victory over world No 7 Alexander Zverev in the last 32 of the Western & Southern Open, which is serving as the warm-up to the first Grand Slam of the Covid-19 era.

The Scot, who said he felt "physically good" after beating young American Frances Tiafoe in the first round in New York in his first competitive match in nine months, backed that up with a 6-3 3-6 7-5 win over the fifth-seeded German, a semi-finalist at the Australian Open earlier this year.

It was a hugely encouraging display from the former world number one, who has admitted has had to lower his expectations since battling injury problems.

Top seed Novak Djokovic was made to work by Ricardas Berankis as he secured a spot in the last 16 at the with a 7-6 (2) 6-4 win.

The world number one was broken in the first set by his Lithuanian opponent but struck back and broke three times in the tie-break to edge in front.

The second set saw more back and forth, with Berankis breaking for a 2-0 lead and again for 4-2, but Djokovic won the last four games on the bounce and will go on to face Tennys Sandgren.

Serena Williams in action in Ohio

Serena Williams and Naomi Osaka only narrowly averted more upsets, both fighting hard in three sets to advance to the round of 16.

After the Sunday defeats of top seed Karolina Pliskova and second seed Sofia Kenin, Williams and Osaka - who both also gained byes into the second round - faced several shaky moments before finally overcoming their lesser-ranked opponents.

Third seed Williams needed to win two tie-breakers to fend off 72nd-ranked Arantxa Rus of the Netherlands, the American ultimately prevailing 7-6 (6) 3-6 7-6 (0) in a tussle 12 minutes short of three hours.

Fourth seed Osaka also had her struggles against 25th-ranked Czech Karolina Muchova, before winning 7-6 (5) 6-4 6-2 in two-and-a-half hours.

Williams will next play Maria Sakkari, after the Greek 13th seed beat Kazakhstan's Yulia Putintseva 6-4 7-6 (9), while Osaka will meet Ukrainian 16th seed Dayana Yastremska, who downed American Bernarda Pera 7-6 (5) 4-6 6-3.

Sixth seed Petra Kvitova, however, became another seed to exit, losing to 48th-ranked fellow Czech Marie Bouzkova 6-2 5-7 2-6.

Fifth seed Aryna Sabalanka of Belarus scraped through against lowly-ranked American Catherine Bellis 6-7 (1) 6-4 7-5, but seventh seeded American Madison Keys was knocked out in her second round match, losing 6-4 6-1 to 39th-ranked Tunisian Ons Jabeur.

In other matches on Monday night, British eighth seed Johanna Konta defeated Kirsten Flipkens of Belgium 6-2 6-0, Belarus's Victoria Azarenka beat France's Caroline Garcia 6-2 7-6 (8), 14th seed Elise Mertens beat Kristina Mladenovic 6-1 6-7 (5) 6-3, and 12th seed Anett Kontaveit of Estonia moved past Jil Teichmann of Switzerland 6-3 6-4.