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No.2 seed Shelton had to battle for two hours and six minutes before eventually prevailing 7-6 (7-4) 3-6 6-3 in his first-round match against an opponent who outplayed his ranking of 112.

Shelton was unable to break Nava in either of the first two sets and then lost his serve in the opening game of the decider as a big upset looked to be on the cards.

But he ultimately recovered and surged to victory as Nava tired in the final set, with Shelton finishing with 14 aces to book a last-16 contest against Alexander Blockx.

"It is tough," Shelton, who made the final in Munich last season, said.

"Not just [playing against] another American, but probably my best friend out on Tour, or one of a few.

"His trajectory has been steep lately, so I’m happy to see the way he has been climbing. I know he wasn’t feeling 100 per cent, so I feel bad about that. I hope he gets back to 100 per cent quickly, because he’s playing great on the clay.

"He has got a massive serve, similar to my first-round matchup [with Borna Gojo] last year, and it made things really difficult." 

Shelton added: "He returned well down the stretch in the second set, broke me when it mattered, and he was clutch.

"So to get through that, I'm really happy. Starting my run here in Munich, it’s a special place for me, and I’m just excited to keep going."

While Shelton went the distance, there were more comfortable straight-sets victories for No.5 seed Francisco Cerundolo and No.7 seed Arthur Rinderknech.

The No.1 seed at the ATP 500 event is home hope Alexander Zverev, who is set to start his campaign against Miomir Kecmanovic, while No.3 seed Alexander Bublik will take on Alex Molcan.

The event in Munich is taking place at the same time as the Barcelona Open, where No.1 seed Carlos Alcaraz is preparing to start his bid for a title on home soil against Finn Otto Virtanen.