We can forensically examine Serena Williams as if she were a unique tennis specimen. We can dissect her serve from yesterday's 2-6, 6-3, 7-6 (8-6) win. Twelve aces to Caroline Wozniacki's none. We can compare winners. Nineteen just on the forehand, which is more than Wozniacki's total of 15.
We can interpret court position, consider fast-twitch fibre and examine her practice ethic. Every little detail matters. But it's not enough. Because what we can't measure is fight.
There is no number for the competitive instinct except the basic one that says 18 grand slam titles. No figures for ego. No statistics for all those things that propel Serena when she's deep into a match. All that sweaty, insane, compelling rage, along with the cold clarity of purpose, which pulls her along like a mountaineer in a blizzard in sight of a summit.
It's this, more than anything else, which Simona Halep will face today in the WTA Finals. This champion stuff which we try to comprehend but never quite do. These voices in the head which urge the extraordinary. This ability not to fear winning but to have the courage to grasp it. This relishing of the moment when a match is raw, uncertain, teetering and there's only the unknown before you.
All this Serena likes.
Yesterday is where you want to inspect great athletes: Right on the fine, sweaty, slippery edge between conquest and collapse. The brave Wozniacki is serving for the match. The athletic Wozniacki is 4-1 up in the third set tie-breaker.
Now what, Serena?
Now, a woman with a big, brassy game, on a dodgy knee, who hasn't played a full match for weeks, on a court where the ball moves slower than glaciers, finds at 33 something more in herself. Because she expects more of herself. Because she believes no one is better than herself. Forget everything else you might think of Serena, you can't get tired of wins like this. Just can't.
There's a mangled Serena racket somewhere in the locker room whose pieces on court had to be carefully picked up by a ball girl. Rafael Nadal never does that, which is extremely decent of him, but sport isn't quite a weekend retreat for saints. Serena said of the racket incident: "Well, at least you know I'm passionate. I give 200 per cent. When I play, it doesn't matter how I feel. I'm going to give everything I have for every shot and every point."
Normally you'd want to roll your eyes. And wonder when athletes are going to come to some consensus on what constitutes maximum effort - 100 per cent, 125, 150 or 200? But this is the only way she knows how to say it because really her finest articulation of her desire is found within the lines. Through shots, not words. If everyone has a quitting point where they go "enough", then hers just seems a little further.
Halep will know all this for it's Serena lore written across three decades. She'll know the Serena numbers, the most recent being that 12 years after first ending the year No.1 she's there again. She'll know the Serena serve, of which Wozniacki said, "when she needed it, it came on fire". And she'll know the Serena pride, which will be smarting from the 6-0, 6-2 schooling that Halep gave her in the round-robin.
"My (first) goal," said Serena in response to that scoreline, "is to win three games (in the final)." You believe that, you probably think Sleeping Beauty is a real-life tale. Serena is a giver of tennis lessons, she's not very adept, like any champion, at taking them. She doesn't want three games from Halep, she wants her scalp.
But Halep, 23, has 10 years on Serena. She has the form. She has a tape of her win over Serena from four days ago. She has the knowledge that Serena has been starting out flat, something even the American said she needs to "figure" out. But she'll have to find what Serena so often does, this faith in crisis, this trust in skill, this commitment to tactic. Serena is not the unbeatable, she just has to be beaten.
No one owns sport. Not even Serena and tennis. Greatness is only leased and then it's taken away. Because without warning someone better has come along on a given day. Halep's got to know this. Got to know that Serena can't keep making history. Got to know that today there is the possibility that she bloody well can.