Serena Williams is braced for a "really tough" US Open showdown with her sister Venus and thinks it is "remarkable" they are both still playing professionally.

The siblings will meet in the third round at Flushing Meadows - and for the 30th time in their illustrious careers - on Friday, 20 years after their first encounter at a major.

Serena is the favourite to win a record seventh title in New York and draw level with Margaret Court's tally of 24 grand slam singles title, while 38-year-old Venus is eyeing a first major triumph for a decade.

Younger sister Serena, 36, is not relishing another family affair as she chases history in her homeland following a 6-2 6-2 second-round defeat of Carina Witthoeft.

"I wouldn't say it's exciting. But it's definitely going to be a really tough match for me." said the 17th seed. "Unfortunately and fortunately we have to play each other. We make each other better. We bring out the best when we play each other. It's what we do, so..."

"I think we're used to it now."

The former world number one did not expect to be doing battle with Venus at the highest level well into their 30s.

"I think that part is really remarkable. I never would have thought that we would still be playing professional," she added.

Venus saw off Camila Giorgi 6-4 7-5 in round two on Wednesday.