Shanghai, China:Britain's Andy Murray sent defending champion Roger Federer crashing out of the Masters Cup in a classic encounter on Friday.
Murray came from a set down to win 4-6, 7-6 (7/3), 7-5 after Federer, apparently hampered by a back injury, saved seven match points in a gripping final set.
The result boosts Murray's chances of winning the eight-man season finale on debut and means France's Gilles Simon also reaches the last four.
"I would have liked to have won on the first match point but I played a couple of bad shots and he hit some aces and some big forehands," Murray said.
"I just had to find a way to come back after that. He was a little bit tired at the end and physically struggling and I had that at the back of my mind."
Federer had never failed to reach the Masters Cup semi-finals in seven straight appearances, winning four titles including the last two.
The 13-time Grand Slam champion, who was forced out of the Paris Masters with a back strain, needed to beat Murray to stay in the tournament after losing to Simon earlier.
Federer was also weakened by a stomach bug in Shanghai, but he started in imperious form and snapped up the first break point of the match at 2-1.
However, Murray immediately broke back after a searing low. The Briton saved more break points when Federer stop-volleyed and rescued an impossible drop-shot, but cracked on set point against his serve.
Murray immediately answered with a break in the next game and another at 4-2, but wasted two set points as Federer broke twice to force a tense tie-break.
With the scores at 3-3, Murray charged ahead and set up three set points with a rare backhand volley before levelling the match at one set each.
Federer twice took treatment for his injured back and he slowed in the third set, using the drop shot whenever possible, and went behind before breaking back with a mighty forehand.
The match was a re-run of this year's US Open final, which Federer won in straight sets. Murray beat the ex-number one in Dubai this year and at last month's Madrid Masters.
Murray has only been beaten once since reaching the US Open final when David Nalbandian ended a run of 14 straight victories in the Paris Masters quarter-finals.
Federer was hoping to kick-start his campaign to win back the number one ranking here. He has endured a difficult year, losing the top spot after 237 weeks and also his five-year Wimbledon streak to Rafael Nadal.
Earlier Simon swept aside Czech alternate Radek Stepanek 6-1, 6-4 but could only qualify when Federer lost.