Doncaster lead Watford 1-0 thanks to Chris Brown's headed goal after Hornets striker Mathias Ranegie had earlier been sent off in a controversial first half at Keepmoat Stadium.
After a quiet opening quarter of an hour the half's defining moment came in the 19th minute as Ranegie became entangled on the ground with Doncaster left back James Husband.
As the Watford striker rose up he petulantly kicked out at Husband and while the linesman didn't flag and the referee appeared to miss the incident, the right decision was eventually made as Ranegie was dismissed.
Spurred on by the extra man, Doncaster began to dominate possession and just five minutes later took the lead as Brown headed home James Coppinger's cross.
Beppe Sannino pushed Ikechi Anya up behind Troy Deeney, presumably with a view to counter-attacking at pace, in a 4-3-1-1 formation.
Watford survived a sustained period of Doncaster pressure but were ultimately unfortunate not to go in all square at the break.
Doncaster were guilty of overplaying at the back and Deeney chased down a weak backpass before blocking the clearance of goalkeeper Sam Johnstone.
The Hornets striker looked certain to get up and tap the ball into an open net but instead he was held back by Johnstone and wrestled to the ground.
To the dismay of the Watford bench and players however, referee Christopher Sarginson awarded neither a penalty to Watford nor a red card to Johnstone and the score remained 1-0 until the interval.