Tsitsipas won his last title in February last year at the Open 13 indoor tournament in Marseille — just weeks before France went into lockdown for the first time because of the coronavirus pandemic.
Monte Carlo Masters: Rafael Nadal 'not worried' about COVID-19 infection after Daniil Medvedev practice
Nadal was not considered a close contact of Medvedev, and therefore not required to quarantine, allowing him to pursue a 12th Monte Carlo title.
Monte Carlo Masters: Stefanos Tsitsipas strolls into last-16; Novak Djokovic to face rising star Jannik Sinner
Tsitsipas, who had been given a bye into the second round, took just one hour and 24 minutes to beat 29th-ranked Russian Karatsev.
Daniil Medvedev will be replaced in the singles draw by Juan Ignacio Londero while being withdrawn from the doubles event.
If the 74 unforced errors were not incriminating enough, Paire's latest post-mortem on tennis in a world of Covid-19 was as explosive as ever.
Rafa Nadal is aiming for a 12th Masters title in the principality ahead of his pursuit of a 14th French Open success and a record 21st Grand Slam.
Nadal has 71-5 record in Monte Carlo where he has fallen short of the quarter-finals just once - on his debut in 2003 as a 16-year-old.
Thiem lost in the last 16 of the Australian Open in February and suffered early exits at both Doha and Dubai.