The 28-year-old Zaheer was dropped from the one-day squad last February due to a form slump and injury, but fought his way back with fine performances for Worcestershire for whom he took 78 wickets in 16 games.
"It (the county stint) helped me grow as a cricketer," Zaheer told Reuters. "You play six days a week and my bowling fitness improved a lot," said the Mumbai bowler who had previously struggled with injuries.
"It helped me get into a routine which is very helpful for your cricket. You get to experiment with a lot of things, and if you are doing things well you are able to continue those things over a period of time.
"It definitely gave me a boost in confidence and county is good, competitive cricket."
The left-armer has rediscovered his best form in time for next month's World Cup following a string of fine displays in South Africa and the recent one-day series against West Indies.
A notably lean-looking Zaheer, who had an outstanding domestic season in India last year, bowled incisively on the South Africa tour, taking 19 wickets in three Tests and four one-dayers.
His return to top form was one of the few gains on a tour where India lost both the Test and one-day series.
He was equally impressive in the four-match series against West Indies which hosts India won 3-1.
Zaheer, who had a stint with Surrey in 2004, said he used the time out of international cricket to work on his bowling technique.
A reduced leap at the point of delivery has helped him retain his pace and enables him to use the crease more.
"I have shortened my run-up and the adjustments I made at the advice of Dennis Lillee (at the pace foundation in Chennai) is bearing results," Zaheer said.
"I had the time to work and experiment with it (bowling technique). And now I have come up with a formula which is working for me. It keeps me in good rhythm."
India play a four-match one-day series with Sri Lanka beginning in Kolkata on Thursday as their final preparations for the World Cup.
Zaheer is confident about India's chances at the showpiece event in March. He was a member of the team that finished runners-up to Australia at the last World Cup in South Africa.
"As long as we do our preparations right, we are a good enough team to beat any side in the world," he said. "We all know that."
