Nick Kyrgios stunned his teammates by pulling out of the United Cup Wednesday on the eve of the tournament, soon after Rafael Nadal said the Australian had "all the weapons" to win a Grand Slam. The temperamental star withdrew ahead of the new mixed-teams event in Sydney, Brisbane and Perth, depriving fans of a scheduled clash between him and the Spanish great. Instead, Alex de Minaur will face Nadal in a round-robin singles match, AFP said. Australia co-captain Sam Stosur said they only learned about Kyrgios' withdrawal minutes before a scheduled team press conference. "We literally just found out," she said. "We move forward now and do our best on the day." The Sydney Morning Herald cited the Kyrgios camp as saying he suffered an ankle injury during an exhibition tournament in Dubai last week. He practiced in Sydney on Tuesday before deciding against competing in the United Cup, they added. Kyrgios' potential has often been betrayed by his on-court behavior, but he had a stellar season this year, reaching his first Grand Slam final at Wimbledon, losing to Novak Djokovic. He was also agonizingly beaten in five sets in the US Open quarter-finals by Russia's Karen Khachanov. Speaking before Kyrgios pulled out of the United Cup, Nadal, who has 22 Grand Slam titles, said the Australian could easily go all the way. The Australian Open next month will be his first opportunity of the year, a tournament where the Spaniard is defending champion. "I know he doesn't like a lot to play Roland Garros, that's the only tournament that looks like he doesn't like to play that much," said Nadal in Sydney. "The rest of the tournaments, he played final in Wimbledon, in New York he was close with the quarter-finals I think, with a very positive chance. "He can win in any tournament that he's playing. He has all the talent, the weapons to win against any player. Of course he can." Kyrgios told reporters earlier this week that he was "training well, enjoying it, mentally feeling good. That's all that really matters". The Australian Open gets under way at Melbourne Park on January 16.