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A back-and-forth contest ultimately turned in the Brazilian's favour as she recovered from a dominant second set from her American opponent to claim the decider and the trophy.
Haddad Maia made a strong start, and took the first set 6-4 after winning 90 per cent of points on her first serve (18 of 20).
Sixth seed Riske came out fighting in the second set, though, breaking in the second game before saving three break points on her own serve, and then breaking for a second time, racing out to a 5-0 lead.
Riske made just five unforced errors in the second set to claim it 6-1 and send the final into a decider.
Seventh seed Haddad Maia was being made to work hard at the start of the third and was broken again by Riske in the third game, but this time was able to break straight back.
Haddad Maia was angered by a line call that went against her in the seventh game, but she used it to find another level, breaking Riske again to take a 5-3 lead, before serving out to seal her maiden WTA singles title.
"It was just amazing. I never expected people here [in Nottingham] cheering for me so thanks a lot for making it special for me," Haddad Maia said in her on-court interview after the win.
"[Riske] improved her game so I tried to stay concentrated... I'm very happy that I was fighting with myself so I could get this trophy.
"It's crazy because I never thought my first [singles title] would be on grass.
"I came here to fight and to win this title, and Nottingham for sure will always be in my heart."