Jannik Sinner sets up semi-final against Novak Djokovic despite 'distressed' fitness signs

Jannik Sinner has defied obvious fitness concerns to set up a semi-final clash against Novak Djokovic after a straight-sets 6-3, 6-4, 6-4 win against Ben Shelton.

The Italian didn't look 100 per cent throughout the match after cramping earlier in the tournament against Eliot Spizzirri.

He noticeably was winding down the clock as long as possible on serve during the second set and Jim Courier expressed concern for the world No.2.

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"You just wonder how much the severe physical trauma he was in during last Friday's match (is impacting him)," Courier said on Nine.

"He had full body cramps, his legs and wrist and hand was going. I wonder if he hasn't fully recovered?"

Later in that set, Sinner sent down a double fault and then uncharacteristically hit a shot wide at 15-15.

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He grabbed at his eyes straight away, which Todd Woodbridge called an "odd look".

"Either he was mainly blinded by the light, your guess is as good as mine but that second serve at 119km/h on the first point, that was odd," he said.

Despite those concerns, Sinner charged on to win the game and set.

When he went to the bench after the second set he put a towel over his face.

But any issues apparently evaporated. With the first point in the third set he hit a massive winner.

"He did look a little distressed at times in that last service game," Lleyton Hewitt said on Nine.

Shelton, the world No.7, kept trying to stay in the match but a late break against his serve cost him and Sinner made no mistake when he was serving for the match.

Shelton's biggest asset throughout the fixture has been his serve. While he didn't win on Wednesday night, he achieved one feat: the fastest serve of the Australian Open.

In the second game of the first set, the left-handed American sent a thunderbolt to Sinner's body, which was returned long.

The speed clock clicked it at 232km/h.

"That's the fastest serve I've seen on Rod Laver Arena ... that was a heater," Courier said.

Overall, Shelton finished with eight aces to Sinner's five.

However, his unforced error count wasn't pretty at 34 with seven of those coming from returns. Sinner had 16 unforced errors but no return unforced errors.

* The following is AI-generated 'What If' analysis that examines the most pivotal points in the match. It was provided courtesy of Infosys

The Zero-Percent Lesson

What if Ben Shelton had buried that forehand at 0-30 in the ninth game of the third set, the very shot that should have been his executioner's stroke?

The American had done everything right, his 176 km/h serve had pushed Jannik Sinner into defensive scrambling, his powerful forehands had stretched the world number one to both corners, yet from that position of total dominance, Shelton's 147 km/h forehand betrayed him, sailing long into the Melbourne night and transforming a potential 15-30 lifeline into a crushing 0-40 deficit.

Down two sets to love and locked at 4-4 in a third set he desperately needed, that unforced error wasn't just a point surrendered; it was the moment Shelton's resistance began to crumble. The cruel irony of tennis: he had won the rally, controlled every exchange, yet one miscalculation from a position of strength would ultimately cost him the game, and perhaps his last realistic chance to extend this Australian Open quarter-final.

The double fault that followed at 15-40 was merely the death certificate; the fatal wound had already been inflicted.

Throughout this quarter-final clash, Shelton found himself trapped in a statistical nightmare that belied the competitive nature of each set. The American generated four break point opportunities across three sets, four chances to crack Sinner's armor, yet converted precisely zero, a 0 per cent conversion rate that will haunt him on the flight home.

Meanwhile, Sinner needed only three of his twelve break point chances to seal a straight-sets victory, the clinical efficiency of a champion who refuses to offer his opponent even a flicker of hope when the finish line beckons.

The physical toll told its own story: Sinner covered 2,093 metres and executed 21 sprints while Shelton's movement data painted a picture of a player increasingly pinned behind the baseline, unable to dictate terms against the Italian's relentless depth.

What makes this defeat so agonizing for Shelton is how close he came to shifting the narrative. In each set, Sinner won three consecutive games to seize control, momentum swings that transformed competitive battles into comfortable leads.

Had Shelton converted even one of those early break points in the first set, had he held serve at 4-4 in the decider instead of gifting Sinner the break with that devastating double fault, the butterfly effect could have been extraordinary.

Instead, the young American exits Melbourne with a lesson etched in heartbreak: against the world's best, the margins between glory and defeat are measured in millimetres and milliseconds, and on this night, every crucial margin belonged to Jannik Sinner.