Bernard Tomic has done his best to scupper suggestions he is a tanker after an epic five-set fightback win in his opening round US Open match on Monday.

After being accused of tanking at last year's tournament, the world No.52 fought back from two sets to one and a break down to win against Spain's Albert Ramos 6-3 3-6 4-6 7-6 (7-1) 6-3 in a tussle lasting almost four hours.

Tomic was in command during the first set, grabbing early control with a flurry of drop shots and powerful forehands to race out to a 5-0 lead.

In the sixth game he took his foot off the pedal to allow Ramos into the set but quickly regained control to take it out in just 33 minutes.

Momentum shifted strongly the Spaniard's way in the second set as he lifted his first serve percentage and broke Tomic to level the match.

In the third set Tomic dropped his service early and was unable to crack Ramos despite numerous opportunities, eventually succumbing 6-4.

And when the Spaniard went ahead a break in the fourth set he was looking down the barrel of another early grand slam exit.

But the Queenslander clawed back a service break and capitalised on the Spaniard's dwindling first serve percentage - and what looked to be a leg injury - to win a one-sided tiebreak 7-1.

In the final set Tomic played smart tennis, moving Ramos around the court before breaking serve in the fifth game.

He closed out the match with another service break, continuing his strong record in five set matches on tour where he has won five of six encounters.

Things only got better for Tomic with the news his expected second round opponent, 11th seed Kei Nishikori, was upset in straight set by unheralded Brit Daniel Evans.