I'd have to nominate Eddie Paynter's 83 in the 4th Test of the now-infamous 1932/33 series as up there with the bravest. Diagnosed with tonsillitis, on the second day's play he was taken to Brisbane General & was found to be running a temperature of 102 degrees. He spent all of the rest day & most of the 3rd day's play in his hospital bed. With England reduced to 216-6 chasing the Aussie's 340 he discharged himself from the hospital (against his doctor's expressed wishes) & took a taxi to the ground.
At the Gabba, fortified by a mixture of egg, brandy & sips of champagne, the little Lancastrian strode out to thunderous applause (his condition having been widely publicised). He refused Woodfull's sporting offer of a runner &, through a trembling "sickly maze" (his own words in his autobiography) added 24 valuable runs in the remaining 75 minutes of play.
Back in the pavilion he donned his pyjamas & was rushed back to hospital where he was greeted with a "Well done; get into bed!" from his ward's sister. After a good night's rest he returned to the ground the next morning, his pockets stuffed with tablets & an antiseptic gargle-mix. He continued this most brave of innings, stopping twice to gargle & take his tablets, eventually falling to Ironmonger with England 16 runs to the good.
He took the field in the Aussies second go, but having reduced himself to the point of exhaustion was forced to return to the hospital after a couple of hours.
It's also pleasant to note that a collection amongst the Gabba crowd for Paynter yielded £40, a small fortune in a time when the average weekly wage for a workingman like him was more like £2.