Novak Djokovic, Serena Williams advance at Western and Southern Open

Facing Lithuania's former junior world No 1 Ricardas Berankis, Djokovic lost break first in each of the two sets, but he broke back immediately and eventually triumphed 7-6(2), 6-4 to improve his perfect record in 2020 to 19-0 on Monday.

Djokovic, who withdrew from men's doubles event with a neck problem, had to call for medical treatment on his neck during the match against Berankis, and he admitted that his neck had an impact on his performance, especially on serve.

"It has bothered me. I made so many double faults, and he knew exactly where I'm going to serve. It's been four days and it's getting better. I think it's heading in the right direction," Djokovic said.

"Hopefully it will not react with the inflammation after today's match. We'll do everything possible with the physio and then we'll see what tomorrow brings," he added.

Elsewhere, Filip Krajinovic of Serbia toppled Austria's second seed Dominic Thiem 6-2, 6-1 while former world number one Andy Murray of Britain defeated world number seven Alexander Zverev of Germany 6-3, 3-6, 7-5.

"It would have been a tough one to lose. I was satisfied obviously to get through it, get a win against a top player after having not played for such a long time," Murray said.

In the women's part, third-seeded Williams took down Dutch qualifier Arantxa Rus 7-6(6), 3-6, 7-6(0).

There were also wins for fourth seed Naomi Osaka of Japan, fifth-seeded Aryna Sabalenka of Belarus and eighth-seeded Briton Johanna Konta.