Thomas, a five-time world champion on the track, was part of a powerful quartet that escaped with 80km to go and eked out a narrow lead to the line.

Slovenian Tadej Pogacar finished safely in the peloton and retained the overall lead of the Giro.

Thomas of Cofidis escaped with Dane Michael Valgren of EF Education-EasyPost, Frenchman Enzo Paleni of Groupama-FDJ and Italian Andrea Pietrobon of Polti-Kometa after an intermediate sprint with less than 80 km to go.

The quartet never led by more than 90 seconds but were able to turn that small margin into victory by working hard together while the sprint teams in the peloton were unable, or unwilling, to organise a coherent chase.

"It was like a long, long team pursuit," said Thomas.

"It was really hard in the finale. Every pull was full gas. With 10km it was still 45 seconds with a tail wind so we knew we could have played with the bunch. We were going over 60 kilometres an hour. That's when we knew we could do it.

"Nobody bet on the breakaway today."

Pietrobon then attacked with almost a kilometre to go and built a lead.

"I risked losing it to win it," said Thomas. "I didn't close the gap. It worked out perfectly."

As the Italian faded, Valgren overtook but was in turn caught on the line by Thomas.

Thomas lives in the region with his Italian partner and rides the roads in training.

"I knew all the road of the final because I train there sometimes. It was helping me to know with the cobbles and the corners," said Thomas. "Nice to win in Italy. Means a lot to me."

The stage was pockmarked by a series of nasty crashes in the nervous peloton.

After seemingly slipping on a manhole cover, French strongman Christophe Laporte rode on with a shredded shirt.

Later, after a group of riders hit a small traffic island, Canadian Riley Pickrell was left with blood pouring down his face.

Finally, Hungarian Attila Valter of Visma hit the ground for the second time in three days.

The rolling 178-kilometre stage from Genoa, with the only serious climb less than a third of the way in, was set up for a mass sprint finish in the hometown of Italy's greatest sprinter, Mario Cipollini -- who holds the record for most Giro stage wins at 42.

However, the breakaway held firm and track cycling star Thomas was the quickest across the line.

The passage through the walled city of Lucca also marked the centenary of the death of composer Giacomo Puccini and the first performance of his last work, Turandot, which contains the most popular operatic aria in sports - 'Nessun dorma'.

Thursday's 180km run to Rapolano Terme starts at a local landmark named after the composer, the Torre del Lago Puccini.