Steve Harmison believes the last day of the third Test against the West Indies is England's biggest of the winter.
The hosts need another 360 runs on Friday to go 2-0 up in the five-Test series with seven wickets remaining.
Fast bowler Harmison told BBC Sport: "We have been in good positions to nail a Test match before and we haven't taken it. This is the big one.
"If you had offered this position when we lost the toss we'd have laughed. It could be phenomenal for us."
The tourists resumed on 31-1 at the start of the day with Alastair Cook's 58 helping them to 221-8 before Andrew Strauss declared with more than a session remaining.
The target of 503 is a huge one to chase for the hosts and the prospect of levelling the series following their demoralising loss in Jamaica is an exciting one for Harmison.
England's paceman, who has been struggling with heat exhaustion and sickness over the last 48 hours, added: "If we can cross the finish line tomorrow it could give us so much momentum and confidence going into the rest of the series and the one-day series."
Strauss's side will hope they have enough time to secure the seven wickets needed for victory, but Harmison supported his skipper's declaration decision.
"We had a set plan this morning," he said. "We were going to bat an hour before tea and then have a good go at them. It was just a coincidence that 500 was the total close by.
"You have to stick to your game plan and we stuck to it very well. Our game plan is to win the Test match and hopefully we'll do that tomorrow (Thursday)."
Harmison revealed he had been "hit for six" with his sickness but was hopeful he was over the worst.
With the Windies having lost Devon Smith - a Harmison leg before, Chris Gayle and Ryan Hinds before the close of play, the Durham seamer knows that the early downfall of Ramnaresh Sarwan (47no) and Shivnarine Chanderpaul (18no) would be crucial.
"It is a massive partnership," Harmison said of the two Guyanese batsmen. "When they are together they are the mainstays of their team. We hope we can knock one over early and put a bit of pressure on."
Chanderpaul's famed concentration will be put to the test on Friday and he revealed he was relishing the challenge.
"We just have to bat to save this game - bat as long as we can and keep them out," said the left-hander. "The wicket is still pretty good, and it is up to us to apply ourselves and fight hard.
"It is a huge total, but cricket is a funny game. If you get a partnership going, we will see what happens."
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