Novak Djokovic won an Australian Open match between two guys with abdominal muscle issues, setting up a semifinal against someone who beat an opponent with debilitating back spasms.

On the other half of the draw, Rafael Nadal's own bad back is improving, at least, as he heads into his quarterfinal against a man whose fourth-round foe pulled out of the Grand Slam tournament because of his own abdominal injury.

So health was top of mind Tuesday night for the No. 1-ranked Djokovic, who sees what is happening all around Melbourne Park and draws a direct connection to the 14-day quarantines imposed because of the coronavirus pandemic when players arrived in Australia in January.

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According to Djokovic, who announced last year he was establishing a new association to represent male tennis players, most don’t want to proceed with the season if it means going through that sort of isolation repeatedly — either with limited time to practice and go to the gym daily or, in the case of those who potentially were exposed to COVID-19 while traveling, barred entirely from leaving their hotel rooms during the two weeks.

"It's too many injuries, honestly," Djokovic said, then sighed heavily, during an on-court interview after his four-set quarterfinal victory against Alexander Zverev.

"I'm just hoping this is all temporary, so we can kind of go back to what we are kind of used to, without interruptions of practice," Djokovic continued. "The 14-day quarantine — people don't realize, but the amount of injuries in this tournament has shown how much effect it has on the players' bodies. It's really — it's taken its toll, unfortunately, on all of us."

Zverev's midsection, like Djokovic's, was taped up for their match. Djokovic's next foe, 114th-ranked qualifier Aslan Karatsev, won Tuesday against 18th-seeded Grigor Dimitrov, who could barely make it up a set of stairs to leave the court afterward and said his back was so painful beforehand it was difficult to put on his socks.

Nadal dealt with a back problem for weeks, sitting out one of the tuneup tournaments held at the Australian Open's site.

No. 9 Matteo Berrettini withdrew after hurting a stomach muscle in the third round. Quarterfinalist Andrey Rublev's fourth-round match ended after two sets when No. 24 Casper Ruud stopped because of an injury. Dimitrov's opponent in the third round, No. 15 Pablo Carreño Busta, stopped after just seven games.

And so on.

"What we are seeing is not normal. It’s not something we are used to. Top players are the ones that are fittest. ... I mean, obviously, it has something to do with these kind of circumstances that we were in," Djokovic said at his post-match news conference, referring to the quarantines meant to prevent the spread of COVID-19.

"I don’t want to sit here (and) complain about what we have been through. But we have to be honest and realistic that it has an effect on the physical well-being of players," he said. "Of course, also mental, emotional."

He said he has "heard a lot of complaints" from players who were willing to go through quarantines for the bigger paychecks at a Grand Slam tournament but are concerned about reduced prize money at lower-level events.

Djokovic also mentioned the possibility of a "bubble" environment of the sort the NBA used last season, with all contests being played at one site — an idea Zverev endorsed.

"We can’t have a traveling circuit right now," said Zverev, who is ranked No. 7 and was the U.S. Open runner-up in September.

"I think what the ATP should do and should look into is maybe having a venue like here and play multiple weeks at one place. Multiple tournaments, multiple weeks," the 23-year-old German said. "Because, at the end of the day, in Europe right now, we can’t have spectators, anyways, so what difference does it really make where we play the tournament? We can change the background, we can change the city name on the court or whatever, and then play it at one venue."

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In response to a request for comment, ATP Chairman Andrea Gaudenzi issued an emailed statement via a spokesman that read in part: "The two-week quarantine was a first, and we are monitoring the impact on player health very closely."

As for the idea of some sort of "bubble," Gaudenzi’s statement said: "The nature of our Tour is truly global — a move away from that structure would present significant challenges compared to many other sports or leagues. We’ll continue to evaluate all viable options to keep the Tour operating and ensure the best possible conditions for players amid today’s challenging circumstances."