A day after crashing into advertising padding during his Monte Carlo Masters first-round loss on Monday, Kokkinakis arrived at the venue on crutches with a leg brace supporting his left knee.

On Friday, agitated German star Alexander Zverev complained that "Kokkinakis broke his knee cap when he fell on one of those" during a fiery exchange with umpire Mohamed Lahyani after tossing away one of the signs during a tough win over Jan-Lennard Struff.

Kokkinakis has yet to publicly disclose the full extent of the injury that has placed the 22-year-old in extreme doubt for next month's French Open.

But his father said Kokkinakis was awaiting scan results.

"Thanasi is in Europe and currently undergoing some tests to assess the impact of his fall. He's staying positive at the moment and we hope to have more information soon," Trevor Kokkinakis said.

Either way, Kokkinakis appears in a desperate race against the clock to be fit for the clay court major beginning in Paris on May 27.

One of only two players to have beaten Roger Federer in 2018, Kokkinakis has just five weeks to recover for the year's second grand slam event.

Missing another major would be a bitter pill to swallow, with Kokkinakis having played just four slams since 2015 due to a catalogue of wretched injuries.

He was limited to just one match in 2016 after undergoing shoulder surgery on his right serving arm before making a promising comeback at last year's French Open.

He has since risen to 150th in the world having plummeted down to 993 in the rankings during his depressing layoff when he contemplated retirement.

Kokkinakis's victory over Federer came last month in Miami after he charged through qualifying before spectacularly ousting the defending champion and 20-time grand slam winner.

Australian Associated Press