Virtanen, ranked 140th in the world, had already battled through three qualifying matches to get to the tournament proper, where his reward was a match with the up-and-coming American, who reached the quarter-finals at SW19 a year ago.
The 25-year-old Finn, by contrast, came into this with only two Grand Slam match wins in his career, both of which came two years ago in the First Rounds at Wimbledon and the US Open.
But a shock looked on the cards late in the first set, when Virtanen sealed a break of serve in the ninth game with his fourth break point, before holding to take the opener. That looked to be as good as it would get for the Finnish hope, as Shelton broke early in the second set, enough to level the match.
However, Virtanen wouldn't lie down, and as both players served well to take set three to a tiebreak, the Hyvinkaa man had chances to take it, but just came up short. As it turned out, he was saving his best tennis for the fourth set, breaking the 22-year-old in the first game and again in the seventh for the most comprehensive set win of the match.
In the decider, Virtanen saved five break points and saw his only one thwarted, and again the pair headed for a tiebreak as the match clock ticked past four hours. A crucial loss of serve from Shelton at 8-8 meant despite giving himself match point at 9-8, it was Virtanen serving to stay in. The Finn not only held, but went 10-9 ahead before an unforced error from Shelton sealed the surprise win for Virtanen.
Virtanen is one win away from a maiden Grand Slam third round appearance. British wildcard entry Arthur Fery will be trying to stop him in the Second Round.
