Sharp, who joined Saints from the Yorkshire club in January, scored in the 58th and 75th minutes to secure a fourth straight win for Nigel Adkins' team and extend second-bottom Doncaster's run to one win in 14.
Reading remain Southampton's closest challengers as they beat play-off hopefuls Blackpool 3-1 at the Madejski Stadium.
The Royals responded well to their midweek defeat at Peterborough and led 2-0 through goals from Ian Harte (30) and Alex Pearce (35) but Lomana LuaLua gave the Seasiders hope with a goal four minutes before the break.
Mikele Leigertwood's strike on the hour mark made sure of the win for Brian McDermott's team, who extended their lead over third-placed West Ham to four points after the Hammers could only take a point at Burnley, their fifth draw in a row.
They looked as though they would leave Lancashire with nothing after falling behind to goals from Marvin Bartley (25) and Martin Paterson (36) but two goals in two minutes from Kevin Nolan and James Tomkins (68 and 70) brought them back into the match.
The gap between third and fourth remains seven points as Brighton were held 1-1 at Nottingham Forest.
Sam Vokes gave the Seagulls a 63rd-minute lead but the sending-off of Matthew Sparrow was quickly followed by Joel Lynch's leveller in the fourth minute of injury time.
Second-placed Reading moved four points clear of West Ham with an impressive 3-1 victory over play-off contenders Blackpool.
The hosts took the lead in the 30th minute when Ian Harte followed up after Jobi McAnuff struck the woodwork, and five minutes later the Royals doubled their advantage.
McAnuff was again the provider, chipping to the back post for Alex Pearce to head home.
Lomana LuaLua pulled a goal back for the visiting Tangerines just before the break with a stunning strike from long range but after the interval Mikele Leigertwood nodded in Reading's third.
Two goals in as many second-half minutes rescued a point for West Ham in a 2-2 draw at Burnley as Sam Allardyce's side fell further off the promotion pace.
Kevin Nolan and James Tomkins secured a fifth successive draw on the back of a dismal first-half showing.
Martin Paterson sent Burnley in two goals to the good at the interval after Marvin Bartley opened the scoring with a fine individual effort.
Defender Joel Lynch equalised four minutes into injury time as Nottingham Forest earned a well-deserved point in a 1-1 draw with 10-man Brighton.
On-loan striker Sam Vokes' goal appeared to have been enough to ensure the Seagulls completed an east midlands double following victory over Derby on Tuesday.
But substitute Matthew Sparrow was sent off for the visitors in the closing stages and the hosts took full advantage when Lynch pounced to poke in from close range.
It was the least Forest deserved after they struck the woodwork three times and created the bulk of the chances.
Bristol City scored their first away goal in 2012 but had to hold on for a 1-1 draw at Middlesbrough after having a man sent off.
Debutant Hogan Ephraim broke the Bristol City duck in the first half with a clever finish before Malaury Martin equalised for the hosts with a powerful effort.
Ryan McGivern was sent off late on for City but Middlesbrough failed to test Dean Gerken, starting in place of David James, as much as they would have liked.
Lloyd Dyer and Ben Marshall struck first-half goals as 10-man Leicester held on to beat Hull 2-1 and keep their play-off hopes alive.
Hull swept into an eighth-minute lead through Matty Fryatt's 11th goal of the season, but Leicester hit back with two superb long-range efforts from Dyer and Marshall in as many minutes.
Foxes midfielder Neil Danns was shown a straight red card in the 57th minute for his reckless challenge on Paul McKenna, but Leicester secured their fourth straight home league win against manager Nigel Pearson's former club.
Ross McCormack fired Leeds to a 1-0 victory at Millwall as Neil Warnock's side put their midweek mauling behind them.
The Elland Road men arrived in south London still smarting from Tuesday's 7-3 humbling by Nottingham Forest.
But McCormack's second-half strike, and a penalty save by Andy Lonergan 60 seconds later, helped banish those memories and secured Warnock's second win since he took charge.
Derby may be able to harbour hopes of gatecrashing the play-offs after a fourth victory in six matches - 3-2 over Crystal Palace - moved them to within five points of the top six.
Palace are also one of a cluster of clubs looking for a late surge to put them into the promotion mix but their 10-game unbeaten league run was ended at Pride Park.
Steven Davies struck early on and Jeff Hendrick scored just before the half-hour mark to give Derby a two-goal lead at half-time.
Theo Robinson's penalty at the start of the second half ensured all three points for Nigel Clough's side despite Chris Martin and Darren Ambrose replying late on for Palace.
Watford hit back to see off Ipswich 2-1 and extend the visitors' long wait for victory over their Hertfordshire rivals.
Substitute Chris Iwelumo was the catalyst for the turnaround, sparking a comeback which inflicted only Town's second defeat in 11 games and left them still without a win over the Hornets for eight years.
Jay Emmanuel-Thomas opened the scoring for the visitors with his fifth league goal of the season in the 18th minute, with the former Arsenal forward capitalising on some hesitant Hornets defending.
However, the home side rallied and Sean Murray followed up Troy Deeney's blocked effort to equalise.
And, with time running out, Deeney's neat lob after a mistake from Carlos Edwards eight minutes from time extended Watford's current hot streak to six games.
Barnsley ended a run of four straight defeats with a 1-0 victory over Peterborough despite playing for almost an hour with 10 men.
The Tykes, on a seven-match winless run and without finding the net in their previous four, took the lead after 24 minutes courtesy of David Cotterill's first goal for the club.
The hosts made things difficult for themselves when Nathan Doyle was dismissed 13 minutes before the break but they held on to pick up a crucial win and stop the rot.
Second-half strikes from Gary McSheffrey and Oliver Norwood helped Coventry boost their survival hopes with a 2-0 victory over fellow strugglers Portsmouth at the Ricoh Arena.
Winger McSheffrey netted his eighth goal of the season 10 minutes after half-time and, after a spell of pressure from Pompey, midfielder Oliver Norwood made the three points safe against the run of play 14 minutes from the end.