Having lost out on the ATP Finals title to Stefanos Tsitsipas last year, Dominic Thiem got his revenge on day one of the 2020 tournament.
Stefanos Tsitsipas faced Rafael Nadal and Dominic Thiem en route to ATP Finals glory last year and will meet both in the 2020 group stage.
Three seeds came through Thursday's play unscathed in Vienna, but Stefanos Tsitsipas was not one of them, instead losing to Grigor Dimitrov.
World number one Novak Djokovic survived a scare in the first set to overcome Filip Krajinovic at the Vienna Open.
Diego Schwartzman ended Dominic Thiem's bid for back-to-back major triumphs, but the Austrian had few complaints.
Diego Schwartzman won a classic Roland Garros quarter-final as Dominic Thiem's hopes of back-to-back grand slam titles were ended.
Rafael Nadal strolled into the French Open quarter-finals, where he will face rising star Jannik Sinner.
Rafael Nadal and Dominic Thiem stayed on course for a semi-final clash, but it was a little-known French player who thrilled Roland Garros.
Rafael Nadal may need to find his best game to land a 13th French Open title, but the Spaniard says tennis perfection cannot be achieved.
Marton Fucsovics gave Daniil Medvedev the Monday blues at the French Open, beating the fourth seed from Russia in four sets.