Even Swiss great Roger Federer believes in miracles after he saved seven match points in a five-set nail-biter against 100th-ranked Tennys Sandgren to labour into the Australian Open semi-finals on Tuesday.

The six-time champion, struggling physically, saved three match points at 4-5 in the fourth set and four more in the tie-breaker to win 6-3, 2-6, 2-6, 7-6 (10/8), 6-3 and set up a clash against either title-holder Novak Djokovic or Milos Raonic.

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Federer received a rare warning for an audible obscenity before going off-court for treatment on a groin injury in the third set, but still looked hampered as he forced a fifth set against the big-serving American.

“I believe in miracles,” Federer said, revealing that he thought his groin strain had ended his chances. “There could be rain, there could be all kinds of stuff. (I thought) just let him finish me off in style, and he didn’t do that. I’m incredibly lucky tonight, today – I don’t even know what time it is.”

Sandgren, achingly close to becoming the lowest-ranked player to beat Federer at the Australian Open, can count himself unlucky especially after a ball girl accidentally collided with him during the pivotal tie-breaker, which he lost.

Federer has made heavy weather of reaching his 46th Major semi-final. He was two points from defeat by John Millman in a fifth-set tie-breaker in round three, and dropped a set against 67th-ranked Marton Fucsovics in round four.

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With inputs from AFP