Great Britain's Andy Murray and Kyle Edmund were swiftly seen off in the first round of the Madrid Open with straight-sets defeats on Thursday.
Having been thrashed by Alex de Minaur in the first round in Monte Carlo, Murray's tough start to the clay season continued as Vavassori prevailed in one hour and 31 minutes.
The writing was on the wall early doors, as Murray was broken to love in the opening game of the match as Vavassori stormed into a 4-0 lead before the Briton began to find some rhythm.
That rhythm came far too late for Murray to save the first set, and Vavassori quickly went a set and a break up in the second, but his counterpart broke back in the eighth game to force a tie-breaker.
In typical Murray fashion, he came up clutch in some vital moments and saved no fewer than four match points, but Vavassori converted his fifth against the irate 35-year-old.
Prior to Murray's defeat, Edmund's run in the Spanish capital came to a swift end with a 4-6 1-6 defeat to two-time finalist Thiem, who took just one hour and 22 minutes to record a straightforward triumph.
Edmund missed two chances to break Thiem's serve in the seventh game, before successive double faults opened the door for the 2020 US Open champion to break in the ninth.
Unforced errors continued to plague the former British number one as Thiem held for a one-set advantage, and an opening hold for Edmund in the second set would be all he had left to cheer.
Thiem capitalised on more double faults and mistakes off the Edmund racquet to win six games on the bounce, beating away another two break opportunities for the Brit and converting his third match point.
Thiem, who will now face fourth seed Stefanos Tsitsipas in the second round, paid tribute to Edmund during his on-court interview, saying: "I have known Kyle since we were juniors. He has been a tough opponent since the young days, so I went in with a lot of respect."
Elsewhere, there were victories for Aryna Sabalenka, Coco Gauff, Jelena Ostapenko and Caroline Garcia in the women's singles, but Miami Open champion Petra Kvitova went down 7-6 6-1 to Jule Niemeier.