Defending champion Jannik Sinner extended his winning run at the Australian Open but young American Alex Michelsen caused the first big upset by knocking out former finalist Stefanos Tsitsipas in the opening round.

Tsitsipas was beaten by Novak Djokovic in the final two years ago and is also a three-time semi-finalist, but he has struggled since reaching the quarter-finals of the French Open last spring.

He has now lost three grand-slam matches in a row after a second-round exit at Wimbledon and first-round defeat at the US Open.

The Greek 11th seed threatened a comeback by winning the third set against Michelsen, but the talented 20-year-old, who reached the third round on his debut last year, held his nerve to claim a 7-5 6-3 2-6 6-4 victory.

“It was a difficult first-round match,” said Tsitsipas. “I knew I was dealing with a pretty serious opponent because I’ve played him before and I have lost.

“So I knew I had to be on top of my game to try and win this match. I had a very slow start to the match.

“In terms of finding my movements and just dominating from the serve plus one, it wasn’t really the way I’m expecting it to work. That led to some frustration and, let’s say, insecurity in terms of approaching my game.”

Tsitsipas admitted he does not know how he will get out of his current slump, saying: “The most frustrating part about losing in the first round of a grand slam is that you have way too much time to recover, and I would rather have the other way around where I don’t have enough time to recover.

“Honestly, that’s much, much better in terms of problem solving. It just sucks in a way that I’ll be hanging around for quite a while now before my next tournament comes in. With my competitive nature, I feel like these type of things are not really ideal for me.”

Sinner has not lost a grand-slam match on hard courts since the 2023 US Open, while his 7-6 (2) 7-6 (5) 6-1 victory over Chile’s Nicolas Jarry made it 15 consecutive victories in all tournaments dating back to a defeat by Carlos Alcaraz in Beijing in early October.

Sinner certainly did not have things all his own way and it took him three sets to break the serve of an opponent who had a bit of extra motivation.

While Sinner waits for the conclusion of his doping case, with a hearing now set at the Court of Arbitration for Sport in April, Jarry was among those left feeling that the Italian had been given preferential treatment.

Sinner was able to play on following two positive tests last March, but Jarry missed virtually the whole of the 2020 season after his own doping violation, for which it was deemed he bore no significant fault or negligence.

Speaking to Chilean newspaper La Tercera, Jarry said: “I would have liked the same support he got when it happened to me. That’s something that affects me personally.”

Tennis authorities have insisted Sinner’s case was dealt with by the book, and he said: “I’m very honest with you. I don’t know the details of his case.

“What I can say also passing these kind of things, we got treated in the same way. There’s the protocol. If the protocol has some issues or problem, then it’s not my fault.

“Of course, I’m very sorry for the players who are passing these kind of things. I know only what happened to me. I knew what I have to pass. It’s basically still on, mine. I have to do some stuff.”

Third seed Carlos Alcaraz came through a few tricky moments to start his season with a 6-1 7-5 6-1 victory over Alexander Shevchenko.

Tenth seed Grigor Dimitrov withdrew through injury while trailing lucky loser Francesco Passaro 7-5 2-1, while Frances Tiafoe vomited mid-match but recovered to beat France’s Arthur Rinderknech in five sets.