Alastair Cook has admitted a sense of disappointment at a lost opportunity despite scoring his maiden Ashes century as England battled to save the third Test.

The 21-year-old Essex batsman hit 116 and appeared to have put the tourists in a strong position by the end of the fourth day at the WACA after sharing crucial partnerships with Ian Bell and Kevin Pietersen.

But Cook, the youngest England batsman in history to score four Test centuries, was one of two victims in three balls for Glenn McGrath just two overs before the close as England slumped to 265 for five and still trail by 291 runs.

It was a big disappointment and Cook, who also scored a century on his Test debut earlier this year against India at Nagpur, who admitted: "It would have been nice to have been only two down and the last few minutes were a bit disappointing. But we're still in there and still fighting hard and if we can get through the first session tomorrow like we did this morning, then who knows?

"I will sit back tonight and know I've got a hundred, but I wouldn't be human if I didn't have that nagging feeling that I could be walking out tomorrow to get an even bigger one and get us close to the target."

Cook shared a crucial 178-run stand with Bell and then added 76 with Pietersen and admitted his innings, spanning six-and-a-half hours in humid heat, tested his endurance. "It's hard work and you don't get a respite," he said. "There are no free runs. "I've felt in quite good nick throughout the series, but I just haven't really got a score. I've hit the ball quite well and it's been frustrating for me to keep getting starts and not going on and I've tried to make it count a little bit."

England finished the day with Pietersen 37 not out while captain Andrew Flintoff had progressed to two. But veteran seamer McGrath believes if they break the current partnership early, Australia can begin celebrating. "We feel we're a couple of wickets away and if we can get them, the Ashes will be home," he said.