Canadian 12th seed Felix Auger-Aliassime has progressed to his first-ever major semi-final after Spanish teenager Carlos Alcaraz's shock withdrawal in the second set of their US Open quarter-final.

Alcaraz, who had won back-to-back five-setters to reach the last eight, retired 68 minutes into Tuesday's quarter-final trailing 6-3 3-1.

The result means Auger-Aliassime, who has never won an ATP Tour title, will take on second seed Daniil Medvedev in the semi-finals on Friday.

The Spanish 18-year-old was attended to by a physiotherapist 2-1 down in the second set and was seen pointing to his upper right leg without receiving any discernible treatment.

Alcaraz had not shown any major signs of injury prior to that yet withdrew following the completion of the next game with the Canadian in control.

Beyond the unusual circumstances, the victory marks a major milestone for 21-year-old Auger-Aliassime, who joins countrywoman Leylah Fernandez in reaching this year's US Open semi-finals.

This marks the second time that two Canadians have reached the semis in the same grand slam, joining Milos Raonic and Eugenie Bouchard at Wimbledon in 2014.

Auger-Aliassime, whose best previous major result was reaching this year's Wimbledon quarter-finals, was strong on his first serve early, delivering 6-1 aces in the first set.

Leading 3-2, the Canadian broke Alcaraz at the third attempt, before both players held to give Auger-Aliassime the chance to serve for the set at 5-3.

In a dramatic game, Auger-Aliassime offered up three break points trailing 0-40, only to rally and win the game to close out the set in 44 minutes, aided by 12 winners.

The 21-year-old broke Alcaraz in the first game of the second set, before Alcaraz's premature retirement, handing the Canadian victory.

Data Slam: First of a new men's generation

Auger-Aliassime becomes the first male born in the 2000s to reach a Grand Slam semi-final, marking a new generation in men's tennis.

WINNERS/UNFORCED ERRORS

Auger-Aliassime – 15/16

Alcaraz – 9/16

ACES/DOUBLE FAULTS

Auger-Aliassime – 8/4

Alcaraz – 1/1

BREAK POINTS WON

Auger-Aliassime – 2/7

Alcaraz – 0/3