Sydney - Austrian top seed Dominic Thiem had a three-set struggle with Portuguese qualifier Gastao Elias before clinching a quarter-final spot at the Sydney International on Wednesday.
Thiem, the world number eight, had several chances to put away the plucky 81st-ranked Elias, only to be broken back before he eventually won 6-7 (5/7), 6-3, 7-5 in a match that finished at midnight.
Thiem will now face Britain's number three Daniel Evans in Thursday's quarters.
"That's three times I've played Gastao and each time it has been very close matches," Thiem said.
"He was running unbelievable with long rallies and I'm very happy to be in the quarter-finals."
Evans fought back from dropping the opening set to down Spanish eighth seed Marcel Granollers 1-6, 6-3, 6-3.
Gilles Muller earlier beat the heat to be one match away from a third straight semi-final appearance at the Sydney International, following a big-serving victory over Australian Matt Barton.
The sixth seed from Luxembourg prevailed over Barton 6-1, 3-6, 6-4 as temperatures nudged 42 Celsius (108 Fahrenheit) to reach the quarter-finals.
Muller, who lost to Grigor Dimitrov in last year's semi-final, will next face Uruguayan second seed Pablo Cuevas, who overcame Frenchman Nicolas Mahut 6-4, 2-6, 6-2.
"It was tough today. I think it was good that we both have big serves, there were not many rallies so that helped to get through it. I was happy when it was over," Muller said.
The 33-year-old added that he was still struggling with the lingering effects of illness from last week's Brisbane International, where he lost in the first round to American Jared Donaldson.
"The result is actually not so bad, the quarter-finals, so I should be happy," he said.
"I like it here. The courts are quite fast, I enjoy the city. I feel good here. I don't know what it is, but it's true my results have been great here over the last three years."
German fifth seed Philipp Kohlschreiber knocked out Australian Jordan Thompson 7-5, 6-4 and will face the two-time defending champion Viktor Troicki.
Third seed Troicki from Serbia won his 13th consecutive match at the Sydney international with a 6-3, 6-4 win over Italy's Paolo Lorenzi.
Kohlschreiber needed just 84 minutes to cruise past Thompson, who failed to save a single break point.
Spanish fourth seed Pablo Carreno-Busta dropped the first set before overcoming German Mischa Zverev 3-6, 7-6 (7/4), 6-3.
Carreno-Busta will face Andrey Kuznetsov in the last eight after the Russian beat Australian teenager Alex De Minaur on an injury retirement at the start of the second set after winning the opening set 6-2.