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Altmaier was staring down the barrel of a defeat in his home tournament in Hamburg as Shelton led 5-4 at the end of the second set.

Yet the American failed to serve out the victory, and Altmaier instead fought back to prevail 4-6 7-6 (7-4) 6-4 and reach the quarter-finals.

"If you see this match, it's a crazy comeback," Altmaier said.

"I'm proud. I stuck to my game. I felt it was a little unfair being a set and 4-5 down, because I felt I was playing quite well. But I'm glad to get through in the end."

At the Geneva Open, Casper Ruud bounced back from his Italian Open final defeat to Jannik Sinner by beating Jenson Brooksby 6-3 7-5.

"This stretch of tournaments is quite a lot but they are nice tournaments and I like playing here," Ruud said.

"I like playing on clay. I try to use the clay season the most I can and every time I come to Geneva I have a good result at Roland-Garros, so let's hope to keep that tradition going."