He does that at most of her matches, finding things to focus on so Williams can keep improving. Yes, that's right: Williams already owns 16 Grand Slam titles, is ranked No. 1, is three weeks from her 32nd birthday — and yet she still wants to get better.
"She's open to discussion, changing things," Mouratoglou said. "She's very open to working hard and doing things that she needs to do."
To that end, Mouratoglou and Williams will sit together and watch video of her matches, going over the positives and negatives he picked out while courtside. He'll refer to his notes, which might involve tactics or technique. She speaks up, too, of course.
"She's working like if she was 19 years old and she wanted to win her first Grand Slam. She has the same appetite, and she's open mentally the same way to get better every day. So that makes a huge difference," Mouratoglou said. "How many players ... have the same appetite? And especially after winning so many Grand Slams?"
Williams is two wins away from her second consecutive U.S. Open championship and fifth in all. Having dropped a total of 13 games through five matches — including a 6-0, 6-0 victory in the quarterfinals — she next faces No. 5 Li Na of China, the 2011 French Open winner. No. 2 Victoria Azarenka of Belarus, the two-time Australian Open champion and last year's runner-up in New York, meets 83rd-ranked Flavia Pennetta in Saturday's other semifinal.
There's little doubt who the favourite is.
Compare Williams' career Grand Slam haul to other three semifinalists' combined three major titles. Or note that Williams already has played in 23 past Grand Slam semifinals, more than twice the 10 previous appearances for the other trio.
Williams began working with Mouratoglou shortly after a first-round loss at the 2012 French Open, the only time she's exited a major tournament after her opening match.
Immediately, he knew one aspect of her game he wanted to fix.
"Seeing that match, and seeing her practice a few days after, I just saw that she was lacking balance. She was unbalanced most of the time. The body weight was going everywhere, except where it should go," Mouratoglou said. "So it was tough for her to be hitting well, the way she would like to — to be precise, to have her usual power."
Once that was solved, he added, "it would be time to do some other things."
Seems be paying off.
Since being beaten at Roland Garros by Virginie Razzano, who was ranked 111th at the time, Williams has won 96 of 101 matches and earned 13 titles. That includes trophies from three of the past five Grand Slam tournaments, along with gold at the London Olympics.
In 2013 alone, she is 65-4 and has matched her career high with eight titles.
Never satisfied, Williams came to the U.S. Open with a new approach: She and Mouratoglou find something new to focus on each and every match.
"Just this tournament, I started setting different goals for myself, and it's been really working for me," Williams said. "So I just started that, and I'm enjoying it."
Explained Mouratoglou: "We just say, 'Well, during this match, you have to do a certain amount of this, a certain amount of that. You have to be able to achieve this.'"
It might be something as straightforward as a target number of forehand winners. Another example Mouratoglou gave was that Williams might set a goal of getting to the net at least a specific number of times, which forces her to prepare points to allow that.
Mouratoglou was asked how Williams is faring with those goals so far at this U.S. Open.
"Not bad at all," he said, smiling. "Everyone sees that she has more tools today. Many more."
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