Jan-Lennard Struff's 7-6(7/4), 6-4, 6-1 win over Hubert Hurkacz of Poland in Sunday's final match saw Germany narrowly avoid their first relegation from the Davis Cup's elite group for 13 years
Andy Murray was leading 6-3, 3-0 when the 40th-ranked Mayer started to play more aggressively, going for winners on nearly every shot. Murray's lead evaporated as he only won three of the last 15 games in the one hour, 51 minute match.
In Saturday's semifinals, del Potro will play doubles specialist Edouard Roger-Vasselin of France, who beat Daniel Brands of Germany 6-3, 4-6, 6-3.
Andy Murray, the reigning Olympic and Wimbledon champion, will next face Uzbekistan's world number 65 Denis Istomin, who put out Italian 20th seed Andreas Seppi, 6-3, 6-4, 2-6, 3-6, 6-1.
Mayer double-faulted to lose the 29-minute opening set and never made an impression on the top seed.
Tomic lost to German Florian Mayer. He will now have to hit the practice courts after missing a chance to build up some valuable hardcourt experience before the start of the US Open in three weeks.
The Swiss top seed dug deep to claim the third set against
Monfils had 16 aces to set up a meeting with sixth-seeded Florian Mayer, who defeated another German, Andreas Beck, 6-4, 3-6, 6-1.
Frenchmen Michael Llodra and Paul-Henri Mathieu have joined seven players who pulled out on Wednesday.
Top-seeded Novak Djokovic eased into the second round of Wimbledon with a 6-3, 7-5, 6-4 win over Florian Mayer of Germany on Tuesday.
He is making his 13th appearance at the pre-Wimbledon warm-up and took the title for the first time in 2012, beating Roger Federer in the final.
Mayer was then made to rue his missed opportunities as after he'd passed up five set points, Murray seized the initiative with thunderous crosscourt forehand at 12-11.
Australian teenager Bernard Tomic admitted on Tuesday that he had given only "85 percent" in his 6-4, 6-0 defeat at the Shanghai Masters, saying the pressures of the tennis tour were taking their toll.
Novak Djokovic brushed aside Florian Mayer in the China Open Saturday to set up a final against Jo-Wilfried Tsonga, as Maria Sharapova and Victoria Azarenka made it to the last two of the women's event.
Novak Djokovic booked a Wimbledon semi-final showdown with Roger Federer as the defending champion crushed Germany's Florian Mayer 6-4, 6-1, 6-4 in the last eight on Wednesday.
Britain's Andy Murray Tuesday defeated Croatia's 16th seed Marin Cilic in their rain-interrupted fourth-round match.
Swiss tennis star Roger Federer has said it is "a joke" that there could be no German men entered in the tennis event at the London Olympic Games.