Dimitrov had already qualified and knew he would face Jack Sock on Saturday, making it all the more important that he spent as little time on court as possible.
And having won the first 11 points of the match, the first set took him just 22 minutes.
The second was more of a contest but the Bulgarian was imperious throughout and need just an hour to dispatch the abject Spaniard.
The prevalence of red and yellow Spanish flags might have been an encouragement to Carreno Busta, but it was also an indication of how many of those tickets might have been sold by the potential presence of Nadal.
It seems unlikely that the world No 1 did not drop his countryman a text a few days ahead of proceedings in London, warning him that he could not practise properly and was unlikely to make it through the week.
But even so, he could be excused the slow start against Dominic Thiem that eventually cost him the match and an already-slim chance at making the semi-finals.
However, there was little to excuse the poverty of his start of Friday, getting himself in trouble with an ill-judged drop shot at 0-15 of his first service game and being broken to love.
He would lose the first 11 points of the match and walked to his chair for the first change of ends break with just one point, courtesy of a Dimitrov netted forehand, to his name.
The Bulgarian was quickly up and into his running, whipping forehands round corners and into open spaces with little Carreno Busta could do to slow him down on the quick surface that is so different from his preferred clay.
The Spaniard has beaten Dimitrov twice on the dirt this year but that never looked likely – the stadium rose as one when he won his first game with 20 minutes gone and trailing 0-5.
But it was merely delaying the inevitable as Dimitrov closed things out and before the half hour was up, more Bulgarian missiles had left Carreno Busta crumpled in a heap at the base of the umpire’s chair having failed to intercept a forehand pass.
Once he had picked himself up, he had to serve at break point down, banging an ace down the middle to his eternal credit. However, Dimitrov was not to be denied and when he did make it three breaks in four service games, Carreno Busta called for trainer to deal with an apparent lower back issue.
It didn’t seem to hinder him too much – and it didn’t break Dimitrov’s rhythm either. The Bulgarian continued to play close to the level that had seen him start the season 16-1 and reach the semi-finals of the Australian Open.
No player had been better behind his first serve at ATP World Tour Finals this week than Dimitrov and so it proved – he lost just one point behind it to Carreno Busta.
With nothing meaningful on the line, it was crucial to Dimitrov that he spent as little time on court as possible, and he ended the match on his terms, breaking for a fifth time to end the match with just two games lost – as he had against David Goffin earlier in the week.
If Sock was watching, it will not have filled him with confidence. Dimitrov is on the charge.