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Shelton, who was defeated in last year's showpiece match by Alexander Zverev, edged closer to another shot at glory in Germany with a 6-4 7-6 (10-8) victory.

The American teed up a blockbuster clash with Joao Fonseca in the next round after the talented teenager dumped out No.7 seed Arthur Rinderknech earlier in the day.

Shelton missed a break opportunity early in the match but was then forced to defend two break points of his own in the sixth game as Blockx looked to get himself in front.

However, the Belgian came undone as Shelton got the better of his serve in a dominant ninth game before closing out the first set to move closer to reaching the last eight.

The pair traded blows throughout a closely-fought second set, with only one break point squandered by Blockx, which would have seen him take the match the distance.

But Shelton regained his composure in a mammoth tie-break to book his place in the next round, sealing his progression with a trademark volley at the net.