Serena Williams
Serena Williams is through to the quarter-finals at the 2018 US Open. In this picture, Serena of the United States is congratulated by her sister and opponent Venus following their ladies singles third round match on Day Five of the 2018 US Open at the USTA Billie Jean King National Tennis Center in the Flushing neighborhood of the Queens borough of New York City, Aug. 31, 2018. Julian Finney/Getty Images

Novak Djokovic heaped praise on Serena Williams and her sister Venus after their third round encounter at the US Open on Friday and admitted he would never be able to do the same if he came up against his brother.

Serena and Venus met in the third round of the US Open — the earliest they have met at a Grand Slam event since the 1998 Australian Open. The early meeting was owing to their low ranking.

It was their 30th career meeting with Serena, the younger of the two Williams sisters holding a 17-12 advantage. The 23-time women’s singles Grand Slam winner went on to make it 18 with a straight sets win over Venus on Friday.

Serena gave birth to daughter Alexis Olympia Ohanian late last year and spent almost 12 months away from the game. She returned and is slowly but surely getting back to her best, which she proved by making the finals at Wimbledon in only her fourth tournament back.

Serena is arguably the greatest women’s tennis player in the history of the sport, while Venus too dominated the game during the last decade. They have a combined 33 Grand Slam titles between them and despite their close relationship off court — they put that aside when on court.

Djokovic admits that he would never have been able to be as aggressive if he came up against either of his two brothers, who do not play professional tennis, on the other side of the court. The Serbian paid respect to Serena and Venus, 36 and 38 respectively, not only for their ability to play each other competitively, but also for their longevity in the sport.

“I have utmost respect for both of them,” Djokovic said after their match on Friday, as quoted on Tennis World USA. “I have two younger brothers. I was kind of imagining what it would be like if I had to play one of them. I mean, I don't know really. Thankfully I won't experience that because I don't know if I would be able to kind of be that fierce competitor against my brother."

“But they manage to do it. I mean, they play, as you say, 30 times. That's something that it's really impressive. As I said, respect. Respect for both what they have done in their careers and to see them play at their age still, keep on going,” he explained. "It's inspirational really for all of us.”

Serena continued her progress at the US Open on Sunday with a three set win over Kaia Kanepi, who knocked out world number one Simona Halep in the first round. She moved into the quarter-finals and will next take on number eight seed Karolina Pliskova.

The American is targeting to become the most successful player in women’s tennis by capturing her 24th women’s singles Grand Slam title, which will take her one above current record holder Margaret Court. Serena’s 23 titles is already a record in the Open Era.