[1] On 28 June 1914, as the chauffeur for Count Alexander von Boos-Waldeck during Archduke Franz Ferdinand and his wife Sophie's visit to Sarajevo, Merz drove the third car in the motorcade.

[2] Merz was the sole driver of the Mercedes Benz SS for the 18 laps of the daunting Nürbrugring Nordschleife, at racing pace, an achievement for which he was widely praised.

This tour de force, his amusing ability to hammer nails through wood with his bare hands, together with his reported attempt to rescue Franz Ferdinand fourteen years earlier, forged the imagery of Metz the colossus, as he became known.

In 1931 he shared Caracciola’s Mercedes-Benz SSKL in the French Grand Prix at Montlhéry, a grueling ten-hour race in the full 12.5 km (7.8 mi) circuit, but the car's supercharger failed after 39 laps.

The six points he scored with Caracciola at the French Grand Prix – the duo completed one-third of the race – tied them for 51st place in the final classification table of the European Championship, won that year by Ferdinando Minoia.

The following year, Mercedes stayed away from the racing circles, and Merz continued to work at the firm as an experimental and test driver.

Taking the 9 km (5.6 mi)-long straights of the Berlin track into consideration, the Mercedes-Benz team produced a streamlined SSKL for the occasion.

Caracciola, who was back to the firm, would be the first choice to drive it, but he was still in the hospital, convalescing from fractures and injuries suffered during a practice accident for the Monaco Grand Prix on 23 April in a privateer Alfa Romeo.

[4][5] A few minutes after 13:00, Merz crashed his SSK on the long straight, overturning[6] near the Grunewald station and nearly 2 km (1.2 mi) from the finish line.

The characteristics of the accident, and the fact it happened in an untested vehicle, has led many experts, including Karl Ludwigsen, to believe the aerodynamic configuration of Merz’s car may have played an important role in this tragedy.