World number 58 Giron upset Evans 6-3 7-5 in one hour and 46 minutes, qualifying for his maiden ATP Tour final.
San Diego native Nakashima progressed into the decider after getting past Australian Christopher O’Connell 6-4 7-6 (7-3).
Nakashima’s triumph means he is into his first tour-level final of the season and it has come in familiar territory.
The fifth seed needed one hour and 50 minutes to beat O’Connell, who had the edge in the key statistics, including 8-7 aces, 34-31 winners and 13-16 unforced errors.
But Nakashima came up big when it mattered, managing the only break of the match in the first set, before triumphing in a tiebreak.
Giron produced the surprise result against world number 25 Evans, who was chasing his first-ever ATP Tour title.
The American sent down 11 aces, while he also hit 40-25 winners. Evans was unable to convert any of the three break points he generated throughout the match.
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