Coco Gauff and Christian Harrison celebrate defeating Greece’s Maria Sakkari and Stefanos Tsitsipas

Perth (Australia) (AFP) - Coco Gauff pulled off double duty to drag defending champions the United States into the United Cup semi-finals on Wednesday while world number two Iga Swiatek did her bit for Poland.

Fourth-ranked Gauff, who suffered a surprise defeat last time out, put the United States on their way in their quarter-final against Greece with a 6-3, 6-2 win over former world number three Maria Sakkari in Perth.

Stefanos Tsitsipas brought Greece level by defeating struggling Taylor Fritz 6-4, 7-5.

In the deciding mixed doubles, Gauff teamed up with Christian Harrison to battle past Tsitsipas and Sakkari 4-6, 6-4, 10-8.

The 21-year-old Gauff struggled with her serve as she suffered her first-ever defeat at the United Cup on Monday when she was stunned by Spain’s Jessica Bouzas Maneiro.

“Definitely a much better match today,” Gauff said after making light work of Sakkari.

“Last match, I just tried to erase it. That’s the beauty of this tournament.

“So definitely happy to give my team the lead today.”

Six-time Grand Slam champion Swiatek beat 97th-ranked Suzan Lamens 6-3, 6-2 in the tie against the Netherlands in Sydney.

Hubert Hurkacz, on his way back after being out for seven months following knee surgery, defeated Tallon Griekspoor 6-3, 7-6 (⁷⁄₄) as Poland sealed their last-eight spot.

Poland will play hosts Australia, with the winner facing the United States for a place in Sunday’s final.

In Perth, Switzerland’a Belinda Bencic beat Solana Sierra 6-2, 6-2 but a 7-5, 6-4 loss by Stan Wawrinka to Sebastian Baez sent the tie into a mixed doubles decider.

The Swiss finished off the win with Bencic and Jakub Paul clinching the winning point over Maria Lourdes Carle and Guido Andreozzi 6-3, 6-3.

The mixed-team United Cup features 18 teams and serves as a build-up towards the Australian Open, which starts on January 18.