Paire retired from his opening match due to "dizziness" when trailing to Norway's Casper Ruud 6-4, 2-0 at the Association of Tennis Professionals (ATP) World Tour 500 Series event.

He was forced to pull out of the US Open after testing positive for COVID-19 and had to spend more than a week in isolation.

Paire's positive test caused issues for organisers of the US Open as those he had come into contact with him had to be placed in what was described as a "bubble within the bubble".

The 31-year-old's return to action came at the Italian Open last week, where he lost n the first round to home favourite Jannik Sinner.

He also played in the men's doubles with compatriot Nicolas Mahut, with the pair suffering defeat in the last 16.

According to German news agency DPA, Paire tested positive twice while in Hamburg, but then had a negative test.

The Frenchman said he was not sure whether he will play the French Open, which starts on Sunday (September 27), and stated he will end his season early if he is not permitted to compete.

Local organisers and the ATP defended the decision to let Paire play.

The ATP said he "was cleared to compete at the tournament in compliance with local health regulations and ATP protocols".

"According to tournament doctor Volker Carrero, it is not uncommon that three weeks after a positive result, fragments of the virus can still be found inside the body," the Hamburg Open said in a statement.

"Paire has not shown any symptoms of disease and has not been contagious at any time.

"Local health authorities in Hamburg made the decision on Saturday (September 19) that Paire is allowed to play."

Top names such as Australia's world number one Ashleigh Barty have decided to miss the French Open due to the threat of the virus.

Japan's Kei Nishikori was one of the big names forced to miss the US Open due to testing positive for COVID-19 before the tournament started.