World number two Casper Ruud crashed out of the Korea Open quarter-finals with 56th-ranked Yoshihito Nishioka on Friday while Britain's Cameron Norrie fell ill.
Japan's Nishioka had the top seed and US Open finalist from the start, breaking her first game on serve without allowing Nora a point before taking the first set 6-2.
Ruud rallied to take the second set 6-3 but was outclassed in the third set as Nishioka won 6-2 to shock in less than two hours in Seoul.
"It was an unfortunate result, but that's how it goes sometimes," said the 23-year-old Ruud, who was also runner-up at the French Open.
“I tried to improve some things and move further up the court, but in the end, it led to too many mistakes.
Ruud defeated Nicolas Jarry of Chile in the last 16 to book his place in the ATP Finals.
Also out is eighth-seeded Norrie,
who pulled out less than an hour before he was due to take the center court to face Jenson Brooksby in the quarter-finals.
Fourth seed Denis Shapovalov is the highest-ranked player.
The Canadian world No. 24 defeated Rada Albot in straight sets and will face Brooksby in the semifinals on Saturday.