Andy Murray dropped into the O2 Arena on Wednesday to watch a little tennis, and was rewarded with the first genuine contest of the singles tournament to date at the Nitto ATP Finals.

Looking at the match-ups in advance, you might not have picked out Marin Cilic v John Isner – electricity pylon against skyscraper – as the most appetising combination.

But tennis is about tension as well as artistic impression, and this was a real arm-wrestle between two of the biggest servers in the game. For Cilic, in particular, his 6-7, 6-3, 6-4 comeback was a cathartic moment, coming after he had won only one of his previous 10 matches at the ATP Finals.

It was a tough result for Isner, whose chances of qualifying for the weekend’s semi-finals are now slim. But he found himself dealing with more serious issues on Wednesday night, as he mourned a friend and former fitness coach: 35-year-old Kyle Morgan, who passed away unexpectedly in the USA.

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Returning to the permutations, Novak Djokovic presumably enjoyed the tightness of Wednesday night’s tussle as much as Murray did. The set dropped by Cilic means that Djokovic – like Kevin Anderson – is now guaranteed to participate in the knock-out phase.

Earlier, Djokovic had been forced to save a couple of break points at 4-4 in the first set of his own match. But that was the end of Alexander Zverev’s resistance. Djokovic reeled off eight of the next nine games, despite a dripping nose and occasional bouts of coughing.

“Unfortunately today hasn't been great in terms of that,” said Djokovic, when asked about his health. “But I managed to gather the strength when I needed it. I think also it seemed like Sascha [Zverev] was not feeling really great. It wasn't a great quality tennis match, to be honest.”

This verdict was supported his opponent. “I haven't felt my best in, like, two months, to be honest,” said Zverev, a 21-year-old who has proved his class by winning three ATP Masters 1000 tournaments over the past two years. “The issue is that our season is way too long,” added Zverev. “We play for 11 months a year. That's ridiculous. No other professional sport does that.”

No sooner had these comments been reported than the former world No 1 Yevgeny Kafelnikov popped up on social media to lambast Zverev. “Fed up with this bulls---,” wrote Kafelnikov on his Twitter page. “He might not remember seasons in 90s, when Grand Slam Cup was ending on Dec 10 and nobody has complained at all, including Sampras, Becker and the rest.”

Yet for all Kafelnikov’s scepticism, most pundits agree that locker-room weariness has been a factor at this year’s ATP Finals. Even after the intervention of Cilic and Isner, the tournament is still on course to surpass its own previous record for uncompetitiveness: the 12 straight-sets victories (out of a total of 15 matches) fought out here in 2015.

Meanwhile the former Wimbledon finalist and world No 2 Agnieszka Radwanska has retired at the age of 29. “I am no longer able to train and play the way I used to,” said Radwanska, whose ranking had dropped to No 75, “and recently my body can't live up to my expectations." In her native Poland, Radwanska’s mastery of the drop shot used to earn her comparisons with Murray.