Arsenal were preparing to train in the sweaty climes of Orlando when Gabriel Martinelli had staff and players checking their watches.

It was last summer’s tour and, for once, the Brazilian’s speed had deserted him. On this occasion, he was slowing everything down.

While Martinelli conducted an interview, team-mates began to playfully heckle the 21-year-old, reminding him the session was starting.

Perhaps they were on to something. Now Martinelli is driving Mikel Arteta’s side to new heights, perhaps this was an early sign that, without him, nothing at Arsenal quite functions properly.

That was the inescapable sense on Sunday. Put simply: Arsenal’s win over Liverpool would not have been possible without the Brazilian winger.

Martinelli scored one goal and created another and it was his tireless running, more than anything else, that drove them towards the Premier League summit. Eight days earlier, against Tottenham, he was again Arteta’s most dangerous player. Something changed either side of that trip Stateside.

This time last year, Martinelli was a promising talent being outshone by other youngsters such as Emile Smith Rowe and Bukayo Saka. This year, he has started every game as Arsenal begin to dream of the title. On current form, he must rank among the Premier League’s finest wingers.

No wonder Arsenal want to tie Martinelli down. Talks are ongoing over a lucrative new deal and on Sunday, he said: ‘Of course, I want to stay. I think it’s my best season here at Arsenal. I’m so happy.’

Yet there has been no spark of sudden change. Rather, Martinelli’s performances are the exhilarating product of more gradual progress. The second half of last season, for instance, was when Arteta’s philosophy really started to crystallise in the Brazilian’s mind. Since then, he has become a more regular fixture on Arsenal’s left wing.

It helps, too, that Arteta’s squad has been strengthened so much in that time. Martinelli and Bukayo Saka are benefitting from the space now created by Gabriel Jesus’ movement and intelligence.

‘He is a great guy, not just on the pitch but off it as well,’ Martinelli said. ‘He is always trying to help me and all the players. He is 25 but he has a lot of experience and everyone loves him.’ That three-way relationship will keep growing. ‘We have a lot of time to improve, I think we have great quality and we can do more,’ Martinelli added.

Oleksandr Zinchenko has helped transform Arsenal, too, and already a relationship is building with Martinelli and Granit Xhaka down Arsenal’s left.

One benefit of being surrounded by greater quality, according to a source, is that Martinelli is now even happier to run himself into the ground as he believes hard work will keep bringing rewards.

Off the pitch, too, things are beginning to click. It is three years since the Brazilian moved to Arsenal as a teenager who could not speak English.

His parents joined him in those early days and David Luiz was a crucial source of advice, too, particularly when up against each other in training.

Now, a source says Martinelli has built his own ‘family’ away from home. He can count on a swelling base of Brazilians, even if Jesus is far better than him at billiards.

And since the summer, Martinelli has struck up ‘a particular bromance’ with Fabio Vieira. ‘They are like two little schoolkids laughing together all the time,’ Sportsmail was told.

Both lean on Cedric Soares, another Portuguese speaker, for support and advice. Martinelli is also close with technical director, Edu. Sometimes, however, there is no greater friend than time. Martinelli arrived aged just 18. Now 21, he is more mature and more settled. No longer, it is said, do the rest of the squad see him as one of the kids.

Staff, meanwhile, are struck by the interest and support he shows them. Among a squad, where Arteta prioritises personality, he remains a ‘standout character.’

‘He has an unusual emotional maturity for his age,’ says one staff member, who claims Martinelli regularly checks on them and asks how he can help. Saka is similar and such awareness is unusual in young players.

In return, Martinelli is ‘absolutely adored’. Assistant coach Steve Round told Edu last season: ‘Honestly, I love him. He trains every day like it’s his last day on Earth. He’s a diamond.’ All Martinelli needed, Round insisted on the All or Nothing series, was time. So it has proved.

And yet, before the recent derby win over Spurs, Martinelli still put faith in fortune.

The winger’s lucky left boot was falling apart but, rather than throw it away, he had it glued and taped back together. By kick-off, however, the damage had become irreparable.

That day, Martinelli toyed with Emerson Royal until the right wing-back saw red. Against Liverpool, he terrorised Trent Alexander-Arnold and Joe Gomez to drag Arsenal top.

Now the winger is making his own luck, how much higher will he — and Arsenal — have risen by their next summer trip?