SHANGHAI (AFP) -- US Open semifinalist Ben Shelton fought back from a set down to defeat sixth seed Jannik Sinner on Tuesday, Oct. 10, and reach the Shanghai Masters last eight in one of the biggest victories of his young career.

The 19th seed emerged from an excruciatingly tight deciding set to battle through 2-6, 6-3, 7-6 (7/5) and faces fellow American Sebastian Korda in the quarterfinals.

The 26th-seeded Korda fended off a late fightback from the 20th seed Francisco Cerundolo of Argentina to go through 7-5, 7-6 (8/6).

Shelton, who lost to Novak Djokovic in the last four in New York last month, did his now-trademark "dialed-in" celebration with an imaginary telephone to mark his first time into the quarterfinals of a Masters.

"I think my toughness level and how resilient I was able to be on the court, I surprised myself a little bit," said Shelton, who turned 21 on Monday.

Shelton endured a torrid opening set against the in-form Sinner, summed up by his double fault to concede the seventh game of an error-strewn first stanza.

The Italian Sinner, fresh from winning the China Open in Beijing, clinched the opening set with a well-placed smash.

Shelton was a changed man in the second, sealing the set with an ace and a roar, and then immediately put Sinner under the cosh to start the decider.

The 22-year-old Sinner survived the initial onslaught, saving five break points to seal a game that lasted nearly 10 minutes, and the set went with serve to force a tiebreak.

Shelton prevailed and a crowd that had been on his side serenaded him by singing happy birthday.

Shelton only turned professional last year and his appearance at the Australian Open in January, where he reached the quarterfinals, was his first time outside the United States.

He is still getting used to the scrutiny that comes with his fast-growing profile.

"It's something that happened quite quickly and was kind of a shock to me," said Shelton, whose father Bryan reached 55 in the world as a player and is now his coach.

Shocks galore

A tournament full of shocks had another to start the day when 91st-ranked Fabian Marozsan had a belated birthday present of his own by beating eighth-seeded Casper Ruud.

The unseeded Hungarian won 7-6 (7/3), 3-6, 6-4 and will face Poland's 16th-seeded Hubert Hurkacz.

"This is just only my second Masters tournament, so it's kind of a new tournament series for me," said Marozsan, who beat Carlos Alcaraz at the Italian Open in May.

"I'm really enjoying this, I'm very proud for this one."

Hurkacz just about avoided falling victim to a shock, defeating unseeded home hope Zhang Zhizhen 7-6 (8/6), 4-6, 7-6 (7/4) in a match that stretched beyond midnight.

The Shanghai event, which is back on the ATP calendar for the first time since the Covid pandemic, has witnessed several shocks.

Korda dumped out second-seeded Daniil Medvedev and Holger Rune and Stefanos Tsitsipas, the third and fourth seeds, are already out.

The upsets have paved the way for top seed Alcaraz, who faces 18th seed Grigor Dimitrov in the last 16 on Wednesday.

World number one Djokovic is not in China.