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 Cristian 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.
Bulgaria's Grigor Dimitrov knocked out third seed and reigning ATP Finals champion Stefanos Tsitsipas 6-7 (5/7), 6-4, 6-3 to record his first top 10 win of the year.
"I was focused, but he went for it," said world number 20 Dimitrov.
"In the tie-break a few close calls here and there went his way. But I kept on believing and kept on doing the right things."
Dimitrov faces Britain's Dan Evans for a semi-final place.