Vacherot landed in Shanghai on a “little bit of a gamble”.
He was on the alternate list for the tournament – meaning he wasn’t even certain to take part – and found out just over a day before the qualifying rounds that he would play.
The performance that followed will propel the Monegasque from 204 to 40 in the rankings, according to the ATP website.
“The challenges were just everywhere,” he said, and his current situation was “surreal”.
On their way to the final, Rinderknech took out world No 3 Alexander Zverev and former US Open champion Daniil Medvedev.
Vacherot ended 24-time Grand Slam champion Novak Djokovic’s chances of a fifth Shanghai title in the semifinals as well as defeating world No 11 Holger Rune.
Vacherot admitted having to face his cousin in the final was “not easy... to deal with”.
Rinderknech broke Vacherot’s serve with a backhand in the third game and it was enough for the 30-year-old to claim the opening set.
Watched on by Vacherot’s idol Roger Federer, neither player was able to make headway early in the second.
Rinderknech faced a break point in the eighth game and saved with an ace.
He found himself on the brink again after an unforced error, and Vacherot did not miss his chance this time to force a decider.
The third set went off with a bang, with Vacherot snatching a break in the first game.
Vacherot looked increasingly dangerous as Rinderknech, seemingly flagging in the humidity, scrambled to save multiple break points in both the third and fifth games.
The Frenchman received treatment on his back after the fifth game, having almost been broken four times.
The partial revival that followed made for some tense rallies that energised the crowd, but Vacherot broke again at 5-3 to seal the title, doubling over in disbelief.
Coach Balleret said that in the second half of the match “the magic” happened.
“[I’ve seen him before] in this state of mind, of physical beast,” he said of Vacherot.
“But to do it on the centre court of Shanghai in the final against his cousin, it’s another thing.”
Rinderknech said after his semifinal that there would be “two winners” on Sunday, whatever happened.
The two sat side-by-side, Rinderknech’s arm around Vacherot’s shoulder, as they waited for the trophy ceremony to begin – and were in tears during it.
Congratulating Vacherot and Balleret, Rinderknech joked: “Two cousins are stronger than one.”
“I gave my everything, I couldn’t have done more, but you deserve it so much, I’m so happy for you,” he said, in a voice that cracked with emotion multiple times.
“I hope there will be more to come, it’s only the beginning.”
“Unfortunately, there was one winner, but probably one family won,” said Vacherot.
“And the sport of tennis won as well, because the story we just wrote is amazing.”
