The 31-year-old is currently in one of the most dominant phases of her career having lost only five matches since the French Open last year.
It was following her shock first-round loss to Virginie Razzano in Paris that Williams began working with Patrick Mouratoglou, and the French coach has received a lot of praise for the impact he has had.
But Williams has said the partnership only works because Mouratoglou has backed up the advice of her father, who is still a key figure in her tennis.
"He's just so innovative, and I think one of the reasons I'm still playing some of my best tennis is because he built my game and my sister's game," she said. "He gave us a good foundation.
"It was solid and it wasn't weak, so we were always able to grow our game. I think that can't really be said about a lot of people in their career.
"He's a great coach. Even to this day. He wrote me something just the other day, and I really meditated on it and I thought about it, and I thought, 'This is what I need to do', and it really helped me out during the match."
Williams has certainly been the most impressive player so far as she bids to defend her US Open title.
The world number one has lost only 13 games in five matches and whitewashed Carla Suarez Navarro in the quarter-finals.
She should have a much tougher task against sixth seed Li Na in the last four tomorrow.
Williams has won their last seven meetings, most recently in Cincinnati last month, but they have all been close.
"She moves really well," said Williams. "I think the challenge of playing someone like her is how do you beat a player that does everything so well?
"I'm going to have to think about that and come up with an answer to that."
Li has had an excellent season and also seems to be benefiting from a different coaching relationship having begun working with Justine Henin's former mentor Carlos Rodriguez last summer.
Previously the Chinesewoman had been coached in various spells by her husband Jiang Shan.
She said of Rodriguez: "At least if he says something I will listen. But if my husband is my coach, when he says something, I never listen. I just say, 'Okay, pass. Next one.'
"So even now if my husband wants to say something, for sure he will say, 'Oh, this is what Carlos says'. I say, 'Okay, okay, I'll listen'."
The other semi-final will pit second seed Victoria Azarenka against surprise package Flavia Pennetta.
Azarenka, who lost a close final to Williams last year, has not been at her best this tournament but comfortably saw off Daniela Hantuchova in the last eight.
The Belarusian was forced out of Wimbledon, coincidentally before a match against Pennetta in the second round, after suffering a knee injury.
She did not play for a month afterwards but believes the lay-off may have strengthened her bid for a first US Open title.
"I work a lot on my fitness," said Azarenka. "I feel like I develop every time I come back, especially after this injury.
"I came back on another fitness level, and my physical ability always helped me to play better tennis. Because my movement is important to me.
"I feel like I have heavier shots now. I direct the ball better. I can reach the balls that I maybe wasn't able to reach because of my flexibility before.
"So little by little it becomes more one piece, and that's what I'm working on. And I'm still going to keep improving."
Pennetta is through to her first grand slam singles semi-final a year after wrist surgery that kept her out of the game for six months.
The 31-year-old is aiming to become the first Italian player to reach a singles final at Flushing Meadows.