Defending champion Daniil Medvedev made a strong start to the Shanghai Masters on Saturday, with his main rival, top seed Carlos Alcaraz, due to launch his campaign later.

Italy's Jannik Sinner is also through after battling through an intense first set -- saving four set points -- against the United States' Marcos Giron.

Medvedev, the world number three, beat Chile's Cristian Garin in straight sets 6-3, 6-3 to proceed breezily to the third round.

Garin put up a valiant defence, with several hard-fought rallies inducing gasps from the crowd, but ultimately the 98th-ranked player struggled with the Russian's relentless pace.

"(There's a) long tournament ahead, with a lot of strong players, starting from my next match," said Medvedev at the end.

"But the goal for sure is to try and win this beautiful trophy one more time."

Medvedev won the 2019 title, but for the next three years the tournament was cancelled because of the Covid-19 pandemic.

"I'm definitely happy to be back in Shanghai," he said. "I honestly feel great and I want to stay here as long as possible."

Medvedev is popular with Chinese fans, a chemistry on display when the indoor match was interrupted briefly after a red-and-white butterfly distracted the Russian from serving.

The crowd giggled as he played up his surprise for comic effect, before carefully handing over the insect, which had settled on one of his tennis balls, to great applause.

"I'm a little bit, not scared of insects, but, like, I don't like when they go on me," Medvedev said at a press conference afterwards, though he added it was "perfect for me" upon hearing they were a symbol of good luck in China.

The Russian next plays 26th-ranked Sebastian Korda, who beat him at their last meeting.

"He was very dominant, in a way, and that's how he plays, so it's going to be interesting for me," said Medvedev.

"I'm in a good shape now, so I will try to do better than last time and try to win."

SINNER PREVAILS

Sinner, who defeated Medvedev in the final of the China Open this week, had a more challenging route to the third round in his Shanghai Masters debut.

The world number four snatched a nail-biting first set 7-6 (9/7), before coming back strong in the second set for a quick 6-2 victory.

"(Giron) is a very, very good player... The first set is never easy, I got lucky a little bit but sometimes you need a bit of luck," Sinner said after the match.

"In the second set I was able to raise the level a bit... so I'm very happy with that."

Fourth seed Stefanos Tsitsipas is also through to the next round after a straightforward job dispatching Australian qualifier Rinky Hijikata 6-4, 6-2.

"In the beginning of a tournament, it takes some time to get the pace and rhythm of how things flow," Tsitsipas said.

"I was very confident and very sure of what I was doing and what I was following and it worked out pretty well."

World number two Alcaraz is also in action Saturday, playing France's Gregoire Barrere.

Alcaraz is looking to close the gap on the absent Novak Djokovic at the top of the rankings with strong performances in China.

But he was disappointed in Beijing by Sinner, who defeated him in the semi-final before going on to beat Medvedev too.