He was worried about Pompey hitting Plymouth on the counter-attack and snatching a late, late goal.
The defender was preparing for extra-time in the 2016 League Two play-off semi-final second leg game.
Up until his intervention, the Blues and the Pilgrims couldn’t be separated, with the Fratton Park clash ending 2-2 and the deadlock unbroken entering stoppage-time at Home Park.
Hartley did eventually opt to jaunt forward for the set-piece – and would infamously etch himself into Pompey history.
The defender’s 91st-minute header found the back of the net – booking Plymouth’s place at Wembley, while piling shear devastation on Paul Cook’s men and the travelling Fratton faithful.
Now at Motherwell, Hartley still vividly remembers those two games – the last time the Blues were in the play-offs ahead of their upcoming League One showdowns with Sunderland.
The 31-year-old lifted the lid on the two fiercely contested battles between Pompey and Plymouth – fuelled by the of-the-pitch rivalry between Cook and opposite number Derek Adams.
And Hartley admitted his goal, which still haunts Blues fans, was one of the luckiest of his career.
He told The News: ‘I remember it all like is was yesterday because it was a big game.
‘We knew we were in for a battle. It literally was a toss of a coin.
‘We thought going into the game that whoever won would be promoted. I'm surmising Portsmouth thought the same and there was nothing in either game.
‘After dropping out of the automatics and into the play-offs, we didn’t go into the games with momentum as we were banking on going up automatically.
‘Genuinely, I thought “here we go, extra-time”. I wasn’t even going to go up for the corner in case we got caught out on the counter-attack.
‘I didn’t think it would be me who’d score – I thought one of the big boys might have got something on it.
'I just gambled and went to the back post and it was a great ball by Graham Carey first and foremost.
‘Goalkeeper Ryan Allsop came for the ball and hesitated and I just remember jumping. I had Enda Stevens marking me and I headed the ball of the back of his head.
‘At the time, I had my eyes shut, and when I opened them the ball hit my head again and went in.
‘I’ve got to say it is one of the luckiest goals I have ever scored! It was great defending by Stevens to get something on it but the ball managed to hit me again.
‘After that I blacked out and jumped straight in with the fans.
'There was a lot of good football played – probably higher than the league deserved to see. Fortunately we came out on top.’
SYMPATHY FOR POMPEY
The Plymouth fans invaded the Home Park pitch at full-time as they set up a Wembley final against AFC Wimbledon.
Pompey, meanwhile, were understandably dejected after being condemned to another year in the Football League’s basement division.
Despite becoming the Pilgrims’ hero, Hartley still had sympathy for the Blues as he was in the exact same position 12 months prior.
'We were in the semis and got beat by Wycombe and I knew exactly how them players were feeling,’ added Hartley.
‘I did feel for them. I remember going up to big Chris Burgess because he was at Hartlepool with me as a young boy.
‘I said to him “go again next year". Because you’re caught in the moment, I knew how it felt so you don’t want to rub it in and keep your distance.
‘In regards of that goal, it's given me legendary status in regards to the Pilgrims.
‘If you can put a lifelong memory in someone's mind with regards to football then it's always going to be a special moment.
‘Even though we didn't get the final result we wanted in the play-off final, to be able to take the fans to Wembley for the first time in 10 years meant it was a special moment.
‘But there were commiserations in regards to Pompey because we were in the same situation the season before.'
“I THOUGHT I WAS GOING TO BE SENT OFF”
The first play-off leg at Fratton Park will long be remembered by those who were in attendance.
There was a bear-pit atmosphere inside PO4, with a thunderous roar created when Marc McNulty opened the scoring in the third minute for the hosts.
Plymouth would quickly turn the tables, though, with Jamille Matt’s quick-fire double putting his side ahead before 20 minutes.
Gary Roberts would grab Cook’s side’s equaliser from the penalty spot – after Hartley brought down McNulty.
The Blues wanted the former Hartlepool United man sent off for the challenge as they felt McNulty would have had a clear goalscoring opportunity.
And if referee Oliver Langford did produce a red card, Hartley wouldn’t have had any complaints at the time.
He said: ‘The atmosphere was absolutely electric at Fratton Park. I felt like I was playing in a top-end Championship game.
‘You couldn't hear anyone on the pitch and the adrenaline got you through the game.
‘Portsmouth fans sucked the ball quite literally into our box because of the roar and noise of them.
'We couldn’t get out of our half in the second 45 minutes and how Pompey didn't score to win the game, I’ll never know.
‘It was devastating to go 1-0 down but we knew with the purple patch Jamille was in then we’d score if he got half a chance.
‘We knew we were going to score. Only in three games all season we didn’t score, so it was just a matter of keeping Pompey out as we knew how good they were attacking-wise.
‘In that moment, I did think I was going to be sent off. But, in hindsight, I do feel the ref made the right call.
‘I had Carl McHugh covering me. When McNulty skipped past me, Carl could have maybe blocked the shot.
‘I feel like it was the right call, but if it was a red card I probably wouldn’t have complained because at the time I thought “oh no". It was a heart-in-the-mouth moment.’
Hartley added: ‘I had a few run-ins with Gary Roberts during the games. I enjoyed it and he's had a great career but I knew he was one of the players who made Pompey tick.
‘My job was to rattle him and I feel like I did.
'He was calling me every name under the sun and I’m not saying I kept him quiet, but if I could leave one or two on him to keep him in his lane then it was job done.’