Jannik Sinner continued his perfect start to his ATP Finals title defence with a scrappy straight-sets win over already-eliminated Ben Shelton.
The Italian won 6-3, 7-6(3) in front of a heavily partisan home crowd at the Inalpi Arena in Turin.
Sinner was not at his sparkling best, struggling on serve, but overcame a spirited opponent to record his 29th straight victory on indoor hard courts.
Having already secured top spot in the the Bjorn Borg group Sinner will avoid Alcaraz, the newly-crowned year-end world No 1, in the semi-finals - with the potential for a blockbuster final clash between the two titans instead.
Sinner will instead face Alex de Minaur - who he has beaten in all 12 of their previous meetings - in the last four, while Alcaraz will take on the winner of Friday’s final round-robin match between Alexander Zverev and Felix Auger-Aliassime.
Shelton made a nervy start but excelled on serve in the second set, going toe-to-toe with Sinner and saving a match point at 5-4 down.
But the defending champion stepped up a gear in the tiebreak to condemn the American to three defeats on his maiden appearance in Turin.
Sinner meanwhile has now won eight straight matches at the season-ending Finals without dropping a set.
Later on Friday Briton Henry Patten and his Finnish partner Harri Heliovaara progressed to the men’s doubles semi-finals with a 7-6(5), 6-2 win over Marcelo Arevalo and Mate Pavic.
The match - a winner-takes-all encounter in the John McEnroe Group - had its fair share of drama, with two controversial flashpoints requiring a video review by chair umpire Fergus Murphy.
Both went the pair’s way. At 4-4 in the first set tiebreak Patten stuck his racquet out in case a Pavic return that was dropping long landed in, with Murphy ultimately ruling the Brit did not touch the ball before it landed. Then at 1-1, 15-15 in the second set Murphy judged Arevalo to have celebrated before the point was over, declaring a hindrance.
Patten and Heliovaara broke two points later, and the second seeds ran away with the set to book their place in the last four for second year in a row.
The duo finished the round robin stage with a 2-1 record and will face home favourites Simone Bolelli and Andrea Vavassori in the semi-final.
The final is guaranteed to feature an all-British pairing, with top seeds Lloyd Glasspool and Julian Cash facing fifth seeds Neal Skupski and Joe Salisbury in the other semi.
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