Complex sentence: The first one to carry the nickname in Formula One was Leo Kinnunen, who wasn't able to turn his success in sports car racing into a successful F1 career in his underpowered Surtees.
Split into simpler sentences: The first one to carry the nickname in Formula One was Leo Kinnunen. However, he wasn't able to turn his success in sports car racing into a successful F1 career in his underpowered Surtees.