Ken Doherty, out of form and struggling for confidence, ignored a worrying lack of recent success to book a quarter-final place in the Benson and Hedges Masters at Wembley last night.But while the world champion was happy to have secured a 6-4 victory over Malta's Tony Drago, he freely admitted that the performance left a lot to be desired."I let Tony off the hook on so many occasions and that's something I've been doing all season," said Doherty. "Of course it's great to get a win like this under my belt but I am not going to get too excited by it."Doherty, whose only title since triumphing at the Crucible last year came at the Malta Grand Prix in November, played in fits and starts with Drago looking capable of scoring his first win in four Masters appearances.In the end, though, Doherty held firm to set up a meeting with James Wattana of Thailand, who earlier in the day roared back from 4-0 down - aided by a trio of century breaks - to edge out John Parrott 6-5."It wasn't a classic but at least I've got over the line," added Doherty. "Now I am looking forward to the rest of the tournament to see if I can do myself justice.Doherty fired in a first-frame break of 89 and recovered from a 42-0 deficit to take the second as well. At that stage, he seemed likely to dominate proceedings but unpredictable Drago had other ideas.A run of 77 gave world number 11 Drago the third frame, he won a scrappy fourth on the brown, and having built up a head of steam, moved 3-2 ahead with contributions of 51 and 50.Doherty rallied to 4-3 but when Drago sprinted through the next frame in a mere nine minutes, the first major upset of the competition began to loom large.Grabbing a disjointed ninth frame allowed Doherty to lead 5-4 though, and despite a few anxious moments he held on in the 10th thanks to making a timely 68 break on his first scoring visit to the table.