Imran Tahir's seven-wicket return put Hampshire in the driving seat at the Rose Bowl but they must bowl Somerset out for a second time if they are to record a vital LV County Championship victory and ease their relegation fears.

The hosts ended the third day still 116 runs ahead, Somerset having followed on 156 runs behind Hampshire's daunting first-innings total of 548.

Somerset, dismissed for 393, were 50 without loss at the close.

Marcus Trescothick, with his seventh championship century of the season, and Craig Kieswetter held up Hampshire's progress for much of the day.

Somerset, 98 for three overnight, made an inauspicious start, losing nightwatchman Alfonso Thomas and Zander de Bruyn to former England all-rounder Dominic Cork in quick succession inside the opening hour.

Thomas (15) was caught behind by Nick Pothas at 129 for four and four runs later Jimmy Adams took De Bruyn at slip to leave Somerset in difficulty at 133 for five.

But Trescothick looked immovable, playing with time and scoring all around the wicket, bringing up his seventh County Championship century of the season and the 84th of his career on the stroke of lunch.

He went on to score 118 - an innings which included two sixes and 14 fours - before being bowled around his outside edge by a ball from Tahir that appeared to turn sharply.

Trescothick's stand of 147 with the prodigiously talented Kieswetter thwarted Hampshire deep into the afternoon session.

Somerset's recovery, seemingly on a steady course at 280 for five with Trescothick's authoritative presence at the crease, lost its direction as Tahir weaved his leg-spin magic to great effect at the pavilion end.

Significantly, Kieswetter was his next victim - superbly caught by Pothas for 94 to leave Somerset 311 for seven, still 88 runs short of the follow-on target.

The Cape Town-raised wicketkeeper-batsman fell six runs short of his second consecutive Championship century, Kieswetter's 94 containing a six and 16 boundaries.

Hampshire were handicapped by the absence for most of the day of their captain and opening bowler Dimitri Mascarenhas, who limped out of the attack with a bruised heel.

His deputy, Pothas, mixed spin with seam, but Tahir was his trump card, having Peter Trego (20) caught before Max Waller joined Omarie Banks (45 not out) in a ninth-wicket stand which threatened the avert the follow-on.

The pair added a lively 70 before Tahir trapped Waller (28) leg before to celebrate his second seven-wicket Championship haul in two seasons. He took seven for 66 against Lancashire at Old Trafford last term.

The stand took Somerset's reply to 392 for nine - and to within seven runs of making Hampshire bat again - but David Griffiths was too sharp for last man Charl Willoughby as the visitors were dismissed for 393.

Tahir finished with figures of seven for 140 from a marathon 35.1 overs and was in the action again, bowling another five overs of leg-spin as Somerset openers Trescothick (26) and Arul Suppiah (20) successfully navigated the final session.