Serena Williams hails Venus Williams at US Open

Serena Williams said she took the emotion out of playing her sister Venus, who - according to the world number one - put up the toughest challenge in her Grand Slam bid so far.

Serena is two wins away from clinching all four majors in 2015, after a 6-2 1-6 6-3 victory on Arthur Ashe Stadium on Tuesday - her 16th win over Venus in 27 meetings.

The 21-time grand slam champion said Venus would have beaten anyone else on Tuesday.

"It's really great to see her do so well. She was at an unbelievable level today," Serena said of her elder sister.

"Down to the match point it just was not easy. It's probably the toughest match I have played in a really, really, really long time where I wasn't actually beating myself.

"I was out there facing an incredibly tough opponent.

"So it was just seeing and knowing that she has that level is so good and inspiring, as well, and hopefully it's encouraging for her, too.

"I think against any other player she - for sure - would have won."

After Venus romped to the second set, Serena said she was not having thoughts that her sister might be about to end her historic attempt.

"I'm thinking, at that point I was glad to be starting out serving," Serena said.

"Just thought, 'OK, I want to hold serve early on and see what happens'. Just, 'What am I not doing? What am I not doing? What can I do better?'

"Nothing different goes in my mind as when I'm playing anyone else."

Standing in Serena's path to the final is Roberta Vinci, a player she has beaten four times out of four - including at the Rogers Cup earlier in the North American swing.

"I played her in Canada. She played me really tough, and I didn't really expect that. That's how I sprained my finger actually, was playing against her," Serena recounted.

"Thankfully my finger is a little better now. But, yeah, so I'm not going to underestimate her. She played really well. 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.

"I think it was really good. Again, I just think it was great that I played her because I kind of know what to expect, and I'll be more ready for it this time."