Australian Open contender Stefanos Tsitsipas struggled with a lower back problem Tuesday, pulling out of his opening singles match of the season at the United Cup as Greece crashed 2-1 to Chile.

The world number six was expected to meet Chilean number one Nicolas Jarry, but sat it out, leaving teammate Stefanos Sakellaridis to step up in the Group B clash in Sydney.

Sakellaridis, just 19 years old and ranked 416, put up an outstanding fight against the 19th-ranked Jarry, but fell short 6-3, 3-6, 7-5.

Greek world number eight Maria Sakkari had earlier earned a commanding 6-0, 6-1 victory over Chile's outclassed Daniela Seguel.

Tsitsipas returned for the crucial mixed doubles decider with Sakkari, but they lost 6-7 (5/7), 6-3, 10-6 to Seguel and Marcelo Tomas Barrios Vera.

"It's amazing, amazing," Barrios Vera said. "It's our second match as a team, so we're super happy."

Tsitsipas, who reached the Australian Open final last year, losing to Novak Djokovic, was clearly struggling on serve late in the match, grimacing as he held his back.

While watching Sakkari's singles, he told the host broadcaster "there are a few chances I might not play and there are a few chances that I might" against Chile.

"I'm kinda 50-50 so far. We'll wait and see."

A six-time Grand slam semi-finalist, Tsitsipas has 10 ATP Tour titles to his name, but is still searching for a first major win.

ALSO READ |&nbsp;<a href="https://www.wionews.com/sports/watch-david-warner-plea-for-return-of-baggy-green-ahead-of-farewell-test-against-pakistan-in-sydney-675710">WATCH: David Warner asks for missing Baggy Green to be returned ahead of farewell Sydney Test</a>

Sakellaridis played the match of his life against Jarry, saving 10 of 12 break points. But his opponent's experience paid off in the end.

"It was an incredible match, very tough for me," said Jarry. "Stefanos played amazing. I think he was grinding a lot, so it was a very tough opponent and I'm happy to be able to be there mentally."

Sakkari, the highest-ranked Greek woman in history, took just 68 minutes to crush Seguel, who was facing a top-20 player for the first time in her career.

She did not face a break point until the last game of the match, and successfully erased the chance.

World number one Djokovic is on court later Tuesday in Perth against the Czech Republic's Jiri Lehecka. Serbia will move into a quarter-final showdown with Australia should they win.

Australian Open contender Stefanos Tsitsipas struggled with a lower back problem Tuesday, pulling out of his opening singles match of the season at the United Cup as Greece crashed 2-1 to Chile.

Australian Open contender Stefanos Tsitsipas struggled with a lower back problem Tuesday, pulling out of his opening singles match of the season at the United Cup as Greece crashed 2-1 to Chile. The world number six was expected to meet Chilean number one Nicolas Jarry, but sat it out, leaving teammate Stefanos Sakellaridis to step up in the Group B clash in Sydney. Sakellaridis, just 19 years old and ranked 416, put up an outstanding fight against the 19th-ranked Jarry, but fell short 6-3, 3-6, 7-5. Greek world number eight Maria Sakkari had earlier earned a commanding 6-0, 6-1 victory over Chile's outclassed Daniela Seguel. Tsitsipas returned for the crucial mixed doubles decider with Sakkari, but they lost 6-7 (5/7), 6-3, 10-6 to Seguel and Marcelo Tomas Barrios Vera. "It's amazing, amazing," Barrios Vera said. "It's our second match as a team, so we're super happy." Tsitsipas, who reached the Australian Open final last year, losing to Novak Djokovic, was clearly struggling on serve late in the match, grimacing as he held his back. While watching Sakkari's singles, he told the host broadcaster "there are a few chances I might not play and there are a few chances that I might" against Chile. "I'm kinda 50-50 so far. We'll wait and see." A six-time Grand slam semi-finalist, Tsitsipas has 10 ATP Tour titles to his name, but is still searching for a first major win. ALSO READ | WATCH: David Warner asks for missing Baggy Green to be returned ahead of farewell Sydney Test Sakellaridis played the match of his life against Jarry, saving 10 of 12 break points. But his opponent's experience paid off in the end. "It was an incredible match, very tough for me," said Jarry. "Stefanos played amazing. I think he was grinding a lot, so it was a very tough opponent and I'm happy to be able to be there mentally." Sakkari, the highest-ranked Greek woman in history, took just 68 minutes to crush Seguel, who was facing a top-20 player for the first time in her career. She did not face a break point until the last game of the match, and successfully erased the chance. World number one Djokovic is on court later Tuesday in Perth against the Czech Republic's Jiri Lehecka. Serbia will move into a quarter-final showdown with Australia should they win.

Tennis: Stefanos Tsitsipas struggles with back issue as Chile stun Greece at United Cup

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Tennis: Stefanos Tsitsipas struggles with back issue as Chile stun Greece at United Cup