Yorkshire are guaranteed a place in the last eight if they beat Northants at Wantage Road tonight (6.30 start).
Lees’s men looked dead and buried at the halfway stage of the competition, when Yorkshire were rock-bottom of the North Group after one win from seven games.
Since then, they have won five out of six and are very much the form team in white-ball cricket, having also won four successive One-Day Cup matches to pull two points clear at the top of the North Group.
It has been some transformation, and one for which Lees, a 23-year-old in his first season as one-day captain, must take plenty of credit.
The job, however, is not yet done as Yorkshire take on a Northants team who are also chasing a home quarter-final.
Northants, in second place, are two points ahead of third-placed Yorkshire Vikings, who could leapfrog them on net run-rate with a victory, depending on the margin.
Should Yorkshire lose, they would still qualify if Birmingham lose to Lancashire at Old Trafford, but if Yorkshire lose and Birmingham win, Yorkshire could be knocked out of the top four on net run-rate if there is a winner of the match between Durham and Derbyshire, who lock horns at Chester-le-Street.
The permutations could drive a teetotaller to drink but the key fact is this – win, and Yorkshire are through come what may.
“We’ve seen a bit of a Lazarus effect in the T20 competition, but now we’ve got to finish the job,” said Lees.
“We’ve played such good cricket now for probably the last month in white-ball, and now we have the opportunity to cement our place and perhaps even to get a home quarter-final, which would be remarkable really.
“We’ve turned it around really well, but we also know that we can’t take anything for granted as Northants have been very good at home and they know a formula and they stick to it.
“They’ve got some very good players – Ben Duckett, in particular, is in great form at the moment – but, if we can get some early wickets and put pressure on their middle order, we could find ourselves in a good position.”
Lees, whose side thrashed Northants by 75 runs at Headingley last Friday, credits his players for the change in fortunes.
In particular, he believes they are performing with great conviction, which has been evident not only in their thumping wins, but also in those tight finishes in which they have had to show bottle to get over the line.
“The lads have done fantastically well,” he said.
“I think a lot of it comes down to a sense of belief.
“We had a poor start to the competition, but then we won a couple of games and we’ve basically taken the confidence from that.
“The lads are believing in each other and everybody’s performing really well.”
Much is made in sport of the word momentum.
It tends to be thrown around like loose confetti, and yet there is no doubting that Yorkshire have discovered it – just as they did in 2012, when they made their sole appearance at Finals Day.
“T20 is such a big confidence game,” said Lees.
“All you need is a couple of wins and you’re off and running.
“Probably the biggest two examples from both ends of the spectrum are ourselves and Worcester.
“I think Worcester won their first few games and can’t qualify now, whereas we lost our first few and can still qualify, so we’ve come into form at just the right time.”
If things go to plan, it promises to be a good 48 hours for Lees and his men, with Yorkshire also guaranteed a quarter-final place if they win their penultimate One-Day Cup group game against Durham at Chester-le-Street on Sunday.
Yorkshire finish off their group campaign with a match against Warwickshire at Headingley on Monday, which the hosts will hope will be purely academic.
“It would be hard for us not to qualify now in the 50-overs,” said Lees. “Mathematically, we could still finish fifth, but playing the cricket we’re playing, I don’t see why we can’t win our last two games and hopefully take those performances into the quarter-finals. As long as we keep believing in each other and respect the fact that we’re not going to win every game of white-ball cricket, hopefully we’ll win the games that matter.
“We’re certainly the form team at the minute in T20 cricket and in 50-overs, and hopefully we can get a good performance at Northants and take that into our one-day stuff on Sunday and Monday.”
Yorkshire squad versus Northants (from): Lyth, Willey, Lees (captain), Head, Leaning, Bresnan, Plunkett, Rhodes, Rashid, Hodd, Rafiq, Patterson, Carver, Coad, Waite.