Isner benefitted as Juan Martin del Potro double-faulted on match point to open the door to the unseeded American, who took full advantage by rallying for a 6-7 (5/7), 7-6 (11/9), 6-3 victory.[break]
Nadal, seeded fourth, gave himself a chance at an ATP-leading ninth title of the season and his second in a row at the Masters 1000 level after his triumph in Montreal last week.
Nadal has defeated Isner three times without a loss.
Apart from six double faults in the second set, world number three Nadal had little to complain about in victory, his 52nd match win of the season.
He is unbeaten on hardcourts, having won the title at Indian Wells in March before his Montreal win.
Nadal came back 18 hours after beating Roger Federer in their marquee quarter-final.
"I have adapted well here," said the Spaniard. "I played a great match with Roger and today I felt I also played well, even if I was a bit streaky with the serve.
"It was a challenge for me today. I never dreamed of this kind of a comeback," said the player who is now in a position to move back to second in the world behind Novak Djokovic if he wins the trophy.
"It´s just unbelievable the way I have come back to be number one in the season points race. I missed the Australian Open, Miami and lost in the Wimbledon first round and still did it," Nadal said.
Nadal fired 38 winners against Berdych, who ousted Wimbledon champion and current world number two Andy Murray in the quarter-finals.
Seventh-seeded del Potro -- bothered occasionally by left wrist pain this week -- never found a way to get back into contention after his double-fault on match point while leading a set and 5-3.
Isner reached the second Masters level final of his career after nearly three hours of battle, turning in a 23-ace performance with 63 winners and 49 unforced errors.
He broke del Potro five times as the Argentine finished with nine double-faults.
"I made that double in the worst moment of the match. If I did it in the beginning of the match or in the third set, it wouldn´t matter. But serving for the match and I made a double-fault," said del Potro, who will no doubt try to iron out the problem before the US Open begins a week from Monday.
"I´m not using that for an excuse, he played better than me in the third set," del Potro added. "He had the chance to beat me, he did very well. He made good serves, good forehands.
"I had the chance to beat him in the second set, but I couldn´t close the match."
Isner was thrilled with his success.
"I can´t believe I´m in the final," said the winner, who plays his best on home courts and is not enamored of the international travel required on the ATP.
Of 10 events this season in the United States, he has reached the semi-finals or better at seven of them.
"The crowd really helped me. Ever after his match point, I knew it wasn´t over until it was over. I just told myself to keep fighting," Isner said.
Azarenka to face Williams in final
CINCINNATI, Ohio, Aug 18: Serena Williams staged comebacks in both sets to defeat China´s Li Na 7-5, 7-5 and reach the final at the Cincinnati WTA tournament for the first time. She will face off for the title against world number two Victoria Azarenka, who won a match plagued by breaks of serve -- 23 in total, including the first six games -- to defeat 2011 finalist Jelena Jankovic 4-6, 6-2, 6-3.
Azarenka and Williams will both be playing for their first title at the tournament.
Azarenka finally earned her victory with a concluding break of former number one Jankovic.
World number one Williams, who has won 15 titles in the last 16 months and is bidding for her ninth this season, needed just over 90 minutes to advance against the error-prone 2012 winner here.
Li ended in unfortunate fashion, double-faulting on a set point in the first and again on match point in the second. Williams claimed her eighth win in nine matches against Li, who had not played in the previous round when opponent Agnieszka Radwanska withdrew to attend a funeral in her native Poland. China´s former Roland Garros winner had a bad day on serve, double-faulting on a set point in the first set and ending the contest after 83 minutes with the same costly error on match point. Williams´ previous best in Cincinnati was a semi-final in 2006 when it was a smaller women´s-only tournament. "When you win a lot you develop a lot of confidence and a belief that you can do well," said the 31-year-old. "I´ve been training more and enjoying it a lot over the past year and a half."
Williams also won the US Open Series title which gives her a chance at a one million-dollar bonus should she win the US Open which begins a week from Monday.
The American admitted to dealing with an abdominal muscle problem this week but did not go into details, saying she did not want to tape it for fear it might hinder her movement. The top seed went up 4-1 in the first set only to be pegged back by Li, who then served for the match but failed. Williams levelled at 5-5 on another of Li´s eight serving errors and then broke again to win the set after 47 minutes. The American then trailed in the second but with Li again unable to serve out. The top Chinese finally went down to defeat after losing serve seven times. The win earned Williams a 60-3 record this season.
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