Both Germany and China advanced to the United Cup quarterfinals after Monday's Group E tie in Perth. Gao Xinyu sealed China's berth with a 6-1, 3-6, 6-3 upset of Germany's Laura Siegemund in 2 hours and 17 minutes, a win that ensured China would finish second in the group. They will face the winner of Group A in the quarterfinals.

However, Siegemund later joined forces with Alexander Zverev to secure a 2-1 win for the defending champions with a 6-2, 7-6(3) victory over Zhang Shuai and Zhang Zhizhen. Earlier, Zverev shook off a sluggish start to give Germany the early lead, rallying to a 2-6, 6-0, 6-2 victory over Zhang Zhizhen.

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Gao's victory continues a dream week for the World No.175, who has replaced the absent Zheng Qinwen in singles. Prior to the United Cup, a professional career that started in 2014 had seen the 27-year-old notch just one tour-level win, in Hua Hin last September, and one defeat of a Top 100 player, over Veronica Cepede Royg in 2017 Beijing qualifying.

Now, including an opening career-best win over Beatriz Haddad Maia, Gao has posted two more of each. Though she had been beset by a leg injury during the 3-hour, 22-minute marathon against Haddad Maia, Gao emerged against Siegemund both moving and swinging freely. By contrast, Siegemund's timing and touch were both off in the first set, during which she committed 14 unforced errors to Gao's seven.

Siegemund raised her level substantially in the second set, going after her forehand and showing off some delightful finesse: a sharply angled half-volley, a pinpoint drop shot-lob combination. She leapt out to a 4-0 lead, and despite Gao responding by retrieving both breaks in succession, managed to hold on to close out the set.

But in the decider, Gao picked up where she left off in the first set, and found her own scorching forehand winner to gain the 4-0 double break. Siegemund clung on valiantly, but Gao stayed clutch, and on match point forced the error from Siegemund with another hefty off forehand.

Zhang had picked up where he left off from his 54-minute clinic against Thiago Monteiro on Friday with a rock-solid start to the winner-takes-all tie. Zverev’s standout quality has been his serving in recent times, but it was thoroughly tested in the opening set against Zhang. The Chinese star broke Zverev on two occasions, and was imperious behind his own, winning 93 per cent (13/14) of first deliveries, according to Infosys ATP Stats.

Zhang, who reached his maiden ATP Tour final in Hangzhou in September, struck cleanly from both wings from the beginning of the match. Yet he was unable to withstand 23-time titlist Zverev once he grew into the encounter and found his range from the baseline. Zverev, significantly aided by the 22 unforced errors that Zhang fired, asserted his dominance with a perfect second-set score.

After a stark drop in intensity in the second set, Zhang steadied himself through the first four games of the decider. But after another lapse in the sixth, Zverev raced away with the final four games to improve to 2-0 in the pair’s Lexus ATP Head2Head meeting.

The final rubber saw Zverev and Siegemund continue their perfect United Cup record together. They had won each of the four previous mixed doubles rubbers they contested at the event, and ensured they kept a clean slate with a rock-solid performance, booking a quarterfinal clash with Kazakhstan.  

“It was a really great match,” Siegemund added. “When you play for the first time after a while, you never really know what’s going to happen, and I think it was a great performance.”