Sinner will face Medvedev in the quarterfinals of the Shanghai Masters

Carlos Alcaraz and Novak Djokovic also made it to the next round after comfortable victories.

But world number three Alexander Zverev is out after losing 6-4, 7-5 to 66th-ranked David Goffin.

Shelton, ranked 16th, knocked out Sinner at the same stage last year, but the Italian looked confident from the start and won in 88 minutes.

The US Open champion prevailed in the ninth game of the first set, taking the lead on a long shot from Shelton and then dominating the second set tiebreak.

World number five Medvedev and Tsitsipas met for the 14th time.

The Greek started the second set strong and broke in the first game, but Medvedev broke in the fourth and eighth sets to secure the win.

Medvedev and Sinner have met four times this year in the final stages of tournaments.

Sinner defeated Medvedev in the semi-finals in Miami, the quarter-finals of the US Open and the final of the Australian Open, but the Russian ended the Italian's hopes in the Wimbledon quarter-finals in an epic five-set battle.

“I will bring the fight,” Medvedev said.

“I could have done much better at the US Open… I didn't fulfill my potential enough in that game. So tomorrow I will try to exploit this potential,” he said.

Sinner said: “We know each other pretty well now, we know more or less exactly what to expect.”

“Of course it will be a very tough game, physically, mentally and tactically, so let’s see what happens.”

World number two Alcaraz, who beat Sinner in the China Open final a week ago, beat veteran Gael Monfils 6-4, 7-5 to secure his 12th straight win.

The 38-year-old Frenchman upset Alcaraz during their last meeting in Cincinnati in August and the Spaniard admitted it was “on his mind” and said he was glad he “stayed calm”.

Alcaraz's quarterfinal opponent is 23-year-old Tomas Machac, who defeated 13th-ranked Tommy Paul 3:6, 6:4, 6:3.

“He has really great tennis. He’s so, so fast too, so I have to concentrate on that,” Alcaraz said of the Czech.

Machac's teenage compatriot Jakub Mensik improved his record against top 10 players, defeating Bulgarian Grigor Dimitrov 6-3, 3-6, 6-4, just days after sending home world number six Andrey Rublev had.

Mensik's next challenge is Djokovic after the Serbian defeated Russia's Roman Safiullin 6-3, 6-2 in 73 minutes.

The former world number one said he had a “great relationship” with Mensik’s family off the field.

“I have always believed that he has the potential to become one of the best players in the world and he is building that momentum,” said the 37-year-old.

He previously played practice sets with the 19-year-old.

“He’s gotten bigger and stronger. So, yes, it will be fun,” grinned Djokovic and expected a “tough challenge.”

In the final match of the day, the tournament lost its second place with Zverev losing in straight sets.

Goffin broke first and kept the pressure on.

Zverev, looking increasingly worse for wear, rallied in the second set but couldn't quite close the gap as the 33-year-old Belgian held on.

“I had nothing to lose, but (Zverev) is always hard to play against,” said an emotional Goffin, who said he decided to revive his career a few months ago.

“It was a great feeling… I didn't expect to be so attacking and relaxed on this court.”

Goffin next faces American Taylor Fritz, who needed less than an hour to defeat 14th-ranked Holger Rune 6-1, 6-2.

reb/dhc