Answer
**Answer**: French Open. **Explanation**: Although Lee and Yoo formed a formidable doubles team, they never managed to secure the French Open title. They came close on two occasions, reaching the finals in 2013 and 2014, but ultimately fell short of victory on both attempts to competitors Marcelo Melo and Ivan Dodig, and Julien Benneteau and Édouard Roger-Vasselin respectively.