(Removes reference to Mertens being top-ranked in second paragraph)
SYDNEY, Jan 8 (Reuters) - Zizou Bergs stunned world number 18 Jakub Mensik and Elise Mertens outplayed Barbora Krejcikova as Belgium beat the Czech Republic 2-0 in the United Cup quarter-finals on Thursday to set up a last-four clash with Switzerland.
Mensik struggled with double faults in the second set as Bergs won 6-2 7-6(4), before Mertens beat Krejcikova 5-7 6-1 7-5 to seal the tie.
Bergs, ranked 42nd in the world, landed four aces as he took the first set in 31 minutes, and kept his cool through a tense second-set tiebreak to clinch his second straight win over a top-20-ranked opponent, having beaten Canadian world number five Felix Auger-Aliassime in the group stage.
"You’re so close, you’re playing big points. I got tight, I go backwards, I play defensive. So it was a very big struggle in the end. I'm so happy for myself and the team," Bergs said.
Mertens wore down two-time Grand Slam winner Krejcikova who committed seven double faults in the third set.
"I felt like I had to play a little aggressive... I'm really happy I could turn it around. It was 1-0 for Belgium, so I could play freely, thanks to Zizou," Mertens said.
Australia face Poland on Friday for a place in Saturday's first semi-final against the United States.
(Reporting by Chiranjit Ojha in Bengaluru, editing by Ed Osmond)
