Australian tennis great Todd Woodbridge has declared the US Open the most physically demanding grand slam, as brutal heat and humidity contributed to a record 12 first-round retirements at Flushing Meadows.

Australian Thanasi Kokkinakis became one of 10 men and two women to retire during matches on Monday and Tuesday – the most in a single round at any major in the 45-year professional era.

The previous record of nine was reached at the 2011 US Open, the second round of 2013 Wimbledon and the opening round of last year’s Australian Open.

Players’ resolve were sorely tested as the mercury shot up past 32C in New York on Tuesday.

Though heat has not officially been blamed for any of the withdrawals, cramps and dehydration were clearly taking their toll on competitors and compounding injury woes.

Kokkinakis suffered severe cramps during his clash with Wimbledon semi-finalist Richard Gasquet on Tuesday.

Despite threatening a major upset the 19-year-old was reduced to under-arm serving before finally calling it quits after falling behind 2-0 in the deciding fifth set, when the chair umpire advise him to consider his health.

Woodbridge said he felt for the teenage ace, recalling how he had suffered his own crippling cramps at the same tournament in the past.

The former doubles champion described the conditions as worse than at the Australian Open in Melbourne, where temperatures regularly hover in the 30s and in 2014 exceeded an inhumane 40 degrees Celsius for four days.

“@usopen most physically demanding of slams due to combo of heat/humidity even more than the 43c days @AustralianOpen,” Woodbridge tweeted.

“Sympathise with @TKokkinakis I full body cramped 1 year @usopen thank heavens for the intravenous drip!”

US OPEN FIRST-ROUND RETIREMENTS:

Men:

Gael Monfils (FRA) retired to Illya Marchenko (UKR)

Yen-Hsun Lu (TPE) retired to Mikhail Kukushkin (KAZ)

Florian Mayer (GER) retired to Martin Klizan (SVK)

Alexandr Dolgopolov (UKR) retired to Samuel Groth (AUS)

Radek Stepanek (CZE) retired to Marsel Ilhan (TUR)

Aleksandr Nedovyesov (KAZ) retired to Lleyton Hewitt (AUS)

Thanasi Kokkinakis (AUS) retired to Richard Gasquet (FRA)

Marcos Baghdatis (CYP) retired to Steve Darcis (BEL)

Ernests Gulbis (LAT) retired to Aljaz Bedene (GBR)

Pablo Andujar (ESP) retired to Teymuraz Gabashvili (RUS)

Women:

Vitalia Diatchenko (RUS) retired to Serena Williams (USA)

Marina Erakovic (NZL) retired to Simona Halep (ROU)