Phil Taylor remained on course to claim a fifth Grand Slam of Darts title by defeating James Wade 16-12 in the quarter-finals at the Wolverhampton Civic Hall.
However, the world No 1 was below his best and was forced to repel a determined fightback from his fellow Englishman before finally wrapping up the victory with an average of 97.89.
The first two legs of the match were shared before Wade missed two darts to break Taylor's throw in the third leg.
Those misses proved costly as it was Taylor who secured the first break in the sixth leg to lead 4-2 and he followed up with a belated first 180 of the match as he held his throw in the next leg.
It was now one-way traffic with Taylor reeling off the following four legs - the highlight a 143 finish in the ninth leg - to lead 9-2.
Wade finally stopped the rot in the 12th leg and that sparked a hot streak which saw him also take the subsequent four legs to close within two legs of Taylor.
Taylor was looking a little rattled and he missed his first five darts at a double in a pivotal 17th leg before finally hitting double two after Wade had also missed three darts for the leg.
The next four legs all went against the darts, but a flustered Taylor kept his nose in front and eventually claimed three successive legs to put himself on the brink of victory at 15-10.
However, he then struggled to get over the line - Wade breaking with a 12-darter - before landing double 40 at the third time of asking in the 28th leg.
Taylor told Sky Sports: "It was a great comeback by James there, I was gone and he got me. He kept clawing back and I kept nicking the odd leg and that's all I did there, I survived.
"That was one of the hardest games I have ever played in my life."
Taylor will now play Adrian Lewis in the last four after the two-time world champion saw off Kim Huybrechts 16-11.
Lewis set up the all-Stoke clash with his former mentor with another solid performance as he averaged 100.16 and hit 12 180s against the in-form Belgian.
The Englishman took control from the start as he landed a 71 finish for an immediate break before holding his own throw.
Huybrechts got off the mark with a 110 finish in the third leg but Lewis made the decisive move by reeling off four successive legs in a run which included a 110 finish of his own and the first 10-dart leg of the tournament.
The Belgian managed to reduce the deficit to 7-5 after Lewis missed four darts at doubles in the 12th leg but that was as close as he got as the world No 3 kept daylight between the pair.
Lewis opened up a 13-7 lead by winning three legs on the trot but Huybrechts responded with finishes of 65, 76 and 148 to stay in touch.
However that run kicked Lewis back into action and an 11-dart leg sparked off his surge to victory.
"I'm delighted to be through because it was a tough game," said Lewis. "I started like a train, but Kim came back well and took his chances.
"Even when he made it 13-10, I always felt I had another gear and I put him back under pressure, but I felt in control for the majority of the match.
"I'm playing really well at the minute, I'm enjoying my darts and playing with a lot of confidence, so I'm looking forward to the semis."