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 No 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 No 1 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 No 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.
Results from the ATP tournament in Vienna on Thursday (x denotes seeding):
Lorenzo Sonego (ITA) bt Hubert Hurkacz (POL) 7-6 (8/6), 7-6 (7/2)
Grigor Dimitrov (BUL) bt Stefanos Tsitsipas (GRE x3) 6-7 (5/7), 6-4, 6-3
Dan Evans (GBR) bt Jurij Rodionov (AUT) 7-5, 6-3
Daniil Medvedev (RUS x4) bt Vasek Pospisil (CAN) 4-6, 6-3, 6-2
Andrey Rublev (RUS x5) bt Jannik Sinner (ITA) 2-1 - retired
Dominic Thiem (AUT x2) bt Christian Garin (CHI) 6-3, 6-2