Russia's Andrey Rublev needed just eight minutes to reach the Vienna ATP quarter-finals on Thursday when Italian teenage opponent Jannik Sinner was forced to quit.
Fifth-seeded Rublev was ahead 2-1 in the first set when the 19-year-old world number 43 retired with a right foot injury.
Rublev, now in his ninth quarter-final of this truncated season, will face second seed and defending champion Dominic Thiem for a spot in the last-four.
US Open winner Thiem eased past Chile's Christian Garin 6-3, 6-2.
Italian lucky loser Lorenzo Sonego defeated Poland's Hubert Hurkacz 7-6 (8/6), 7-6 (7/2) and next plays world number one Novak Djokovic.