Top seed Dominic Thiem and Russian Andrey Rublev progressed to the Qatar Open semi-finals in Doha after grinding out victories over Stefanos Tsitsipas and Borna Coric on Thursday.
Greece's Tsitsipas took the early lead in the first set but Thiem, the only seeded player still in contention for the season-opening title, fought back to win 7-5 6-4.
"I was serving well again today and returning well. That's what I was working on in the off-season so it already pays off now," Thiem said.
Croatian Coric was expected to overcome Rublev, but the 20-year-old Russian won 6-3 7-5 to set up a semi-final against Guido Pella.
Both Pella and his quarter-final opponent Mirza Basic were surprise contenders in the last eight after knocking out top-seeded players and the Argentine world number 64 cruised to a 6-2 6-3 victory over the Bosnian.
World number five Thiem will face Gael Monfils in the last four after the Frenchman beat German Peter Gojowczyk 6-3 7-6.
