The fourth-ranked del Potro missed a chance to win in his first event since a five-set loss to Rafael Nadal in the Wimbledon quarter-finals.
The 2009 US Open champion, who is enjoying a career resurgence after finally shaking off a long-term wrist injury, won 6-3 7-6 (8-6) after one hour and 43 minutes on outdoor hard courts.
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Del Potro, though, took a quick lead, 3-0, before Fognini settled in, ran the big Argentine ragged with his varied shot-making and tactical nous, and rattled off six of the next seven games.
Second-seeded Fabio Fognini of Italy will play British left-hander Cameron Norrie in the other semifinal.
The world number 15 secured the eighth ATP title of his career and third in 2018, having previously won in Brazil and Sweden this year, although it was his first success on hard courts and the first time he has managed more than two tournament triumphs in a calendar year.
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Fognini needed just the one more break to seal the set and then raced away to victory with a rather one-sided second set, dropping just the two games in that set.
He will face either top seed Juan Martin del Potro or number three seed Damir Dzumhur in the final. "I took my chances to win and I'm glad to reach another final this season".
"Unfortunately, I couldn't play my best tennis in the final", Del Potro said afterward. But Fognini settled in, defending from all parts of the court and using his forehand to push Del Potro behind the baseline.
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