With 15 minutes played, and his side already a goal to the good, Thomas was shown the red card by referee Drysdale for an elbow on Saints midfield player John Viafara. That changed the match as the Clarets were forced back onto the defensive and this negated their excellent early start.
For the second successive match Burnley raced out of the blocks and into an early lead as Michael Duff's long throw from the right was not cleared on two occasions by Saints defenders allowing Steve Jones a left-foot finish past keeper Kelvin Davis.
Five minutes later Duff was again involved linking up well with Wade Elliott before supplying a cross from which Gifton Noel-Williams headed wide.
Following the dismissal of Thomas, the Saints were soon level as Chris Makin's cross from the left was guided past Brian Jensen by the head of Rasiak.
Burnley boss Steve Cotterill reacted to the sending off of his skipper by sacrificing wide man Elliott and introducing veteran defender Frank Sinclair to shore up his back four.
The 10 men of Burnley went in front as Sinclair's free-kick was glanced past Davis by the head of striker Andy Gray.
Saints were indebted to Davis for not falling further behind five minutes into the second half when the keeper got a hand to a fine strike from the rampaging Noel-Williams.
The importance of that save was emphasised moments later when Rudi Skacel danced round a couple of challenges before firing beyond Jensen from 25 yards for his first league goal since joining from Hearts.
With George Burley's team enjoying most of the possession it came as no surprise when Southampton went in front for the first time with 16 minutes remaining as striker Rasiak met Skacel's cross with a fine header past the helpless Jensen.
Even with the numerical advantage, Southampton relied on Davis to be alert to keep out a pile-driver from Gray and, deep into five minutes of stoppage time, Davis again produced a fine save to prevent Gray from earning the home side a hard-fought share of the spoils.