VIDEO of Jared Donaldson’s underhand serve: unsporting or genius?

Jared Donaldson produced an underhand serve during his French Open match against Grigor Dimitrov.
The American went down 6-7 (2-7), 6-4, 4-6, 6-4, 10-8 to the third-seeded Dimitrov in the second round.
However, he decided to serve underhand in the fifth set and won the point.
Underhanded??@JaredD takes the lead 7-6 in the 5th.
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He repeated the trick a few games later, but lost that point and eventually lost the match.
Donaldson, of course, also had a run-in with an umpire earlier in the clay-court season.