Van Barneveld beat Phil Taylor in a classic final in 2007
Raymond van Barneveld revealed he is struggling to cope with diabetes after beating Kevin Painter in the third round of the PDC World Championship.
Van Barneveld, the 2007 champion, beat a below-par Painter 4-1 to move into the quarter-finals, but looked close to exhaustion towards the end.
"In the fourth set I was really tired," said Dutchman Van Barneveld.
"In August I found out I had diabetes and I can't handle it. For some reason it's there and you are so tired."
Van Barneveld, the 2007 champion, has only dropped one set in the tournament so far, but he will have to up his game if he is to progress any further.
He eased through the first set before Painter, runner-up in 2004, missed a raft of doubles in the second to hand Van Barneveld a 2-0 lead.
Painter continued to struggle with his finishing in the third as Van Barneveld moved into a 3-0 lead, but Painter recovered from 2-1 down in the fourth to reduce the deficit.
Van Barneveld was dripping with sweat by the end of the fourth set, but he managed to hold it together to secure the victory.
A relieved Van Barneveld will now play Ronnie Baxter, who breezed past Colin Lloyd 4-1.
It is the first time Blackpool's Baxter, who whitewashed Gary Anderson 4-0 in the second round, has reached the quarter-finals since 2002.
Chelmsford's Lloyd missed numerous doubles in the first set to allow Baxter to nick it before racing into a 2-0 lead.
Lloyd had two chances at doubles to win the third set, but once again allowed Baxter to steal it, before Lloyd finally got into the game with a two-dart 74 to make it 3-1.
Baxter just missed out on a nine-darter in the fourth set, but he closed out the match with a double top after Lloyd missed four chances to keep the match alive.
James Wade beat Andy Hamilton 4-3 in a classic encounter to set up a last eight clash with Simon Whitlock, who beat Terry Jenkins 4-2.
Wade recovered from 3-2 down to claim victory over Hamilton after his opponent missed a dart for the match.
Hamilton hit maximums in the first two legs and while Wade hit back, Hamilton won first set with a 140 checkout.
Wade, the world number three, levelled with a clean sweep in the next set and then edged the third with a 65 under pressure while his opponent was waiting on a two-dart finish.
Hamilton threw some great darts to level at two sets each and regained the overall lead when he took the fifth with a double 10. However, Wade eased to the sixth set to send the match into a decider.
The fourth leg saw Hamilton have a dart for the match, but he failed to hit double 18 to complete a memorable 156 checkout and Wade held his nerve to send the match into a tie-break.
Wade won the fifth leg with double top and then converted a 112 checkout to claim victory.
Whitlock had already beaten seeds Colin Osborne and Wayne Jones before meeting Jenkins, and he soon eased into a 2-0 lead against the world number six.
Jenkins reduced the deficit with a double six, but Whitlock edged a similarly-tight fourth set by nailing a 61 finish to lead 3-1.
Jenkins again hit back to keep his hopes alive, but Whitlock overcame a 2-1 deficit in the sixth set to move into the quarter-finals for the first time.
Wales' Mark Webster beat Kirk Shepherd 4-1 and will now play Dutchman Co Stompe, who beat Mark Dudbridge 4-2.