#1 Ivan Lendl
With eight Grand Slam titles, seven Year End Championships and 94 Singles titles, Ivan Lendl was already a legend in tennis annals.
But the taciturn Czech didn't stop there. Post-retirement he took up coaching responsibilities and most notably had two stints as coach of Britain's Andy Murray. Prior to hiring Lendl, Murray had faced a series of setbacks in Grand Slam tournaments falling at the last hurdle to either Federer, Nadal or Djokovic.
Lendl had a huge role in Murray's transitioning to being one of the contenders to actually being a Grand Slam champion. He helped Murray win his first Grand Slam major at the US Open of 2012 following his heartbreaking loss to Federer in the final of Wimbledon the same year.
Following year, he coached Murray to the Wimbledon title helping him achieve a lifelong dream. It was deja vu all over again for Lendl as Murray, just like him triumphed in a Grand Slam final in his 5th attempt. In 2014, the two parted ways only to reunite in 2016. Again their partnership worked wonders as Murray won Wimbledon once again, rose to the top of the ATP rankings for the first time and retained his Olympic Gold Medal at the Rio Olympic Games. In 2017, they again parted ways
Prior to the US Open of 2018, Lendl took over as coach of Alexander Zverev and coached him to his maiden ATP Finals trophy in London.