Late in the second set, Garbine Muguruza hits an acutely-angled backhand cross-court and approaches the net. Agnieszka Radwanska, she of the noiseless game, responds with a backhand slice passing shot that slithers cross-court for a winner. It is a piece of such cool geometry and audacious beauty that it merits a place in a sports museum. Quite simply it deserves to be seen by everyone.
La Profesora is what Muguruza called Radwanska the other day and yesterday bandaged Polish professor and exhausted Spanish student produced a lively tennis debate with 11 breaks of serve. The crying Pole remains in the event but the Spaniard has left an indelible impression.
Muguruza this week has been what most folk wish for from sport: pure fun. Graceful in speech, quick with a smile, stylishly erect in carriage, low to the ground for her shots, devoted to risk, desperate for victory. Chris Evert, careful with her praise, was clear the other day: "I think she's got a great attitude."
But by yesterday Muguruza, who also played doubles all week, was a race car running on fumes. The WTA Finals are not long but they are suffocating in their intensity, for great rival in one round is followed by a greater rival in the next. As she collapsed onto her chair in the press room later, she was asked if she was tired.
"Oh yeah," she smiled.
This will probably not be Muguruza's finest year and yet, in a way, it may always be her purest year. Because the innocence of 2015 will be lost when the calendar turns in January. This year she announced her talent, next year it will be expected.
Muguruza is entertaining and dangerous because she expects every shot, however difficult, to go in and often they do. Errors are brushed off like cheap lint from a dress. Backhand in net is followed by backhand screaming for winner. As her Twitter profile makes it clear: "Because life's just too big to play small!"
And if yesterday she got her equation slightly wrong - 42 winners but 61 unforced errors - then she is already making a muscular impact. When the Pole was asked to choose a nickname for the Spaniard, she came up with this: The Bomb.
"Because," said Radwanska, "she's very explosive, especially from every shot. So forehand, backhand, serve, return, everything is coming to you so fast."
This will probably not be Muguruza's finest year and yet, in a way, it may always be her purest year. Because the innocence of 2015 will be lost when the calendar turns in January. This year she announced her talent, next year it will be expected. As Evert noted: "She has all the makings of a No. 1 player; whether she'll be No. 1 is another question."
And yet it is precisely this pressure that champions wait for and it is exactly this position that Muguruza has worked for. As Joan Solsona, Spanish tennis writer with the Marca newspaper, says: "It is not a case where she thinks 'Oh, it's such a surprise to be here'. Step by step she has worked to get here and now she is working to be No. 1."
Height, hunger, heft is not Muguruza's only advantage. So is environment. She arrives from Spain, replete with footballers and basketballers and Fernando Alonso and so the spotlight doesn't just burn her. In their shadow, she can train; through their feats, she can be inspired. Indeed, in her fellow citizen Rafael Nadal is to be found a rare role model of sweat and spirit.
"Garbine is playing amazing," said Nadal yesterday and he, the unmerciful, will like the fact that in her first WTA Finals she beat the world No. 5 Petra Kvitova, No. 7 Angelique Kerber, the No. 9 Lucie Safarova.
But yesterday, in a match of hastily switching momentum, her mistakes were greater than her will. And, of course, she was playing Radwanska, an experience which must seem like trying to solve a crossword puzzle while on the move.
Both sparred creatively - drop shots in response to drop shots - but Radwanska just seemed to have a little more. When asked later if she surprises herself with her shot-making, her face dissolved into a smile: "Yeah, sometimes like, 'Oh, my God. It's in'!"
Today Radwanska confronts another violent player, Kvitova, in the singles final while Muguruza will be a supporting act in the doubles final.
"I learn," the Spaniard said later of her year "that I'm able, in the bad situations, to calm down... clear my mind and start from zero. I learn that I'm more strong than I thought in these moments."
We, meanwhile, are still learning to say her name.
Practise it. In 2016, we'll need it.