London - England captain Alastair Cook believes the team's best limited overs days are ahead of them even though they are now again the world's top-ranked side in 50-over international cricket.
Cook's men briefly lost their place at the summit of the standings after a heavy defeat by South Africa in Southampton last week.
But they bounced back in London following wins by four and six wickets in the one-day internationals (ODIs) at The Oval and Lord's respectively.
Sunday's success at Lord's gave England a 2-1 lead in a five-match campaign heading into Wednesday's finale in Nottingham and meant South Africa could no longer achieve the series victory they needed to extend their brief stay at number one.
Meanwhile Australia's recent defeat by Pakistan in Abu Dhabi scuppered their hopes of overtaking England at the head of the standings before the year is out.
"It's certainly not a bad thing," Cook, who last week also became England's Test captain after the retirement of Andrew Strauss, said of being number one.
"But I think we feel this side has its future ahead of it, rather than behind it," explained the 27-year-old opening batsman, who will hope to lead England to a maiden World Cup title in Australia and New Zealand in 2015.
Cook, a member of the England side that lost its position at the head of the Test standings to South Africa earlier this season, added: "We're still striving to improve in all areas.
"We've only been together for a year or so. We've had some really good results, but certainly don't feel like the number one side -- like we did when we got there as a Test team."
England suffered an 80 run-hammering in Southampton but bounced back well.
Sunday's win at Lord's saw them, albeit after Cook won the toss in overcast conditions, restrict South Africa to 220 for eight.
They then overcame the early loss of Cook to make 224 for four with man-of-the-match Ian Bell, revelling in his new role as a one-day opener, scoring 88 and putting on 141 with Warwickshire colleague Jonathan Trott (48).
"Clearly after what's happened this summer, we could have let our heads go down -- a bit of a case of 'here we go again'," said Cook.
"We admitted we didn't get it right at the Ageas Bowl (in Southampton), and we've bounced back really well.
"We've still got a series to win, but I'm very proud of what we've done this weekend in back-to-back matches."
Bell replaced Kevin Pietersen as an opener after the South Africa-born batsman briefly retired from limited overs internationals.
Pietersen has since rescinded that decision but finds himself in England exile over issues of "trust and mutual respect".
He was dropped following his 149 in the drawn second Test against the Proteas last month after it emerged he'd sent "provocative texts", some allegedly critical of his team-mates, to opposition players.
But whatever the Test ramifications, Bell's form means Pietersen has not been missed in one-dayers.
"It was a classy knock again. I say that time and time again when you watch Belly play," said Cook.
"He has so much time, and he's delivering the goods at the top of the order. He's come back into the side, and he's been outstanding."
Meanwhile South Africa captain AB de Villiers, disappointed with the way his side had failed to score enough runs for the second consecutive match, was pleased to still have the goal of a series-levelling victory in his sights.
"I'd like to think the whole of the top six play as a unit and we haven't been performing well," said de Villiers. "There's no excuse, the guys are batting in the places they feel comfortable.
"The positive thing is the series is still alive. We can level it, which would be a great achievement, and I really believe we can do that."