Rune, leading 2-1 in the first on serve, and with just a quarter of an hour of match play gone, saw his opponent make the walk to his chair and call it quits. Tsitsipas quickly made up his mind during the first changeover and a brief consultation with the ATP physio.
Tsitsipas later confirmed he had been struggling with a back injury and was given the "green light" to play by the doctors. But it was clear he was not going to make it through the match and had to retire and withdraw from the tournament.
For the Dane, this was an early Christmas present. He got the win that gives him hope of qualifying for the semifinal and will need to defeat Jannik Sinner to progress, potentially in straight sets.
Rune is guaranteed lucrative pay for 17 minutes of work. This year's ATP Finals is offering a record prize money of up to $15 million and this will be reflected in all rounds at the tournament beginning with the round-robin stage.
There is a participation fee of $325,500 for a player who will play group matches. Rune earned $162,750 after playing in his first match, where he lost to Novak Djokovic on his debut. By stepping on the court to face Tsitsipas, he pocketed $81,375 in participation fee.
Each round-robin win accounts for an impressive sum of $390,000. Rune's earnings for playing and winning that second group match (via retirement) took his day's earnings to $471,375, which on average is almost $28K per minute. He was on court for 17 minutes.
Rune will now switch his attention to the final group match where he is scheduled to play Jannik Sinner on Thursday in a must-win clash.