The 36-year-old saved the first of them, but former top-20 junior Joint’s aggression paid dividends one last time.
The Australian young gun nailed an inside-out forehand return then sent her dashing the other way before completing the boilover with a drive backhand volley winner.
A disbelieving Joint instantly turned to her courtside team, who she embraced after meeting Siegemund at the net. Up next will be American 14th seed Madison Keys or Czech Katerina Siniakova.
Joint’s breakout year includes reaching final-round qualifying at the Australian Open, becoming the youngest Australian at 17 to win a $75,000 event at Burnie, another ITF title in Santo Domingo, a maiden top-100 win over Anna Karolina Schmiedlova at a WTA 125 in June, plus a first WTA 125 final in Poland in July.
Playing as a seed at a major for the first time, Popyrin maturely dealt with the odd spot fire on his way to dispatching South Korean Soonwoo Kwon 7-5, 6-2, 6-3 in barely two hours to reach the second round.
It was a solid start for the towering Australian at the world’s most boisterous slam, where players deal with a never-ending stream of planes flying overhead and a generally more raucous atmosphere in New York.
Alexei Popyrin advanced to the second round at the US Open.Credit: Getty Images
The Montreal champion has never had so much expectation on him since his breakthrough Masters 1000 triumph a fortnight ago – where he beat five consecutive top-20 rivals, including Andrey Rublev in the final – propelled him inside the top 30 for the first time.
Popyrin blasted 14 aces, lost only three of his 37 first-serve points, and hit almost double as many winners overall (37-20) as Kwon, who hurt his own cause with 13 double faults and often at the worst times.
They traded breaks in the opening set before Kwon book-ended the 11th game with double faults to gift Popyrin the break and soon a one-set lead.
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But the key moment came early in the second set, when another multi-double fault game from Kwon handed Popyrin a break point. Another wild forehand from the South Korean sealed the break, but he was in a great position to repeat his first set effort of breaking back immediately.
Popyrin sunk to 0-40, but reeled off the next five points, including pounding a forehand winner to go 3-1 up. Another break in the seventh game helped the Australian snatch a two-sets advantage.
There was little between the players through the first six games of the third set before Popyrin made his move, crushing a superb backhand return winner as Kwon attempted a rare serve-volley then cruised to victory.
The news was not so good for Gold Coaster Birrell, who lost 6-4, 6-4 to Wimbledon semi-finalist and Olympic silver medallist Donna Vekic.
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