Isner won the opening five games of the match as he secured the opening set 6-1, but was pegged back as Gulbis broke his serve in the second game of the second set to take an early lead. Gulbis then served out to take the set 6-3 and level the match.
Neither player had a chance to break again until the seventh game of the decisive set, as Gulbis raced to 0-40 on Isner's serve. The Latvian then converted his third break point opportunity to edge in front, before he broke Isner again to seal the third set 6-3 and win the match.
Gulbis, who is currently ranked No. 145 in the world, has not won an ATP Tour title since 2014, when he was ranked as high as No. 10. Gulbis also defeated Isner's fellow American Jack Sock in the quarterfinals.