Venus Williams believes sister Serena is the best female player of all time with or without the calendar grand slam.

Serena beat Venus 6-2 1-6 6-3 to reach the US Open semi-finals and now stands two victories away from becoming the first player to hold all four major titles in a year since Steffi Graf in 1988.

It was the 27th all-Williams clash but, despite a mid-match lull where Venus staged a fightback, Serena proved why she has been the dominant force in recent years with a ruthless display.

The world number one managed 12 aces, 35 winners and three breaks of serve under the lights of Arthur Ashe Stadium and on this form, it seems inconceivable she will not be writing her name into the history books after Saturday’s final.

“I think that would be huge, not just for me, but for my family, just for what it represents and how hard we have worked and where we come from,” Venus said.

“So it would be a moment for our family. But at the same time, if it doesn’t happen it’s not going to make or break you.

“We don’t have anything to prove. She has nothing to prove. She’s really the best ever, so what are you going to do?

“Just try to make it. If you don’t, then that’s that and go to the next one.”

Serena would be only the fourth female player to achieve the calendar grand slam and success at Flushing Meadows would also see her tie with Graf’s Open era record of 22 major titles.

“Serena is the best ever because of the level of competition that she’s faced,” Venus said.

“There have been some unbelievable players in the past, but I have played in what seems like multiple eras at this point. I have played the best from different eras, as well.

“I have seen the level of competitiveness go up, and I have seen players who are ranked 100, who didn’t believe they could win a match against you, fight you tooth and nail and try to take you down.

“That didn’t happen when I started so just to be able to win at this level, I think that’s what makes her the best.”

The sisters shared a warm embrace at the net after an hour and 38 minutes of action and Venus, who whispered ‘So happy for you’ in her sibling’s ear, insists there was some consolation in defeat.

“Losing isn’t fun,” she said. “But probably the most gratifying thing is I’m still very excited to see Serena have an opportunity to win the four majors. I think that’s the best part.”

Serena usually contains her emotional body language during battles with her older sister but the 33-year-old was visibly fired up throughout the contest.

“What was the best bit? Walking off the court and it being over with,” Serena said.

“It’s probably the toughest match I have played in a really, really, really long time where I wasn’t actually beating myself.

“I think against any other player she for sure would have won.”

Williams now faces unseeded Italian Roberta Vinci, who ousted Kristina Mladenovic 6-3 5-7 6-4 to reach her first grand slam semi-final at the age of 32.

The pair have met on four previous occasions and Vinci is yet to take a set off the American.

“I’m not going to underestimate her,” Serena said.

“She’s not in the semi-finals of a grand slam for no reason. She knows what to do and she knows what to play.”