O'Sullivan began the 1998–99 season by winning the non-ranking Scottish Masters, but went on to win no more titles during the season. At the UK Championship, O'Sullivan was defending champion, but withdrew from the tournament shortly before his scheduled first-round match. His manager stated that he was "suffering from physical and nervous exhaustion" and that his doctor had ordered "a complete rest from snooker". Later reports stated instead that he was suffering from depression at the time. In the quarter-finals of the Welsh Open, O'Sullivan made the second maximum of his professional career, playing against James Wattana. O'Sullivan reached the quarter-finals of the Masters, but lost 2–6 to Ken Doherty. In his fourth consecutive final in the Charity Challenge, and in a repeat outcome of last year's final: O'Sullivan lost to John Higgins again, this time 4–9. At the World Championship, O'Sullivan reached his third semi-final in four years. But, he was again denied a place in the final when he lost 13–17 to Stephen Hendry in a high quality match that featured century breaks in four consecutive frames (two by O'Sullivan of 134 and 110, scoring 134 because of missing the pink for the maximum break).