Sir Michael Stoute is well aware the Queen's filly Estimate faces the toughest test of her career in today's Gold Cup at Royal Ascot.

The four-year-old daughter of Monsun has won three of her six career starts and heads to the 2½-mile test as one of the least experienced members of the 18-runner field.

Nevertheless, she is a likely favourite, having provided Her Majesty with a win at the meeting last year in the Queen's Vase as well as impressing on her seasonal reappearance.

John Oxx's Saddler's Rock should possibly have a few more big-race wins on his cv than just the Doncaster and Goodwood Cups. Not a horse seemingly blessed with a huge amount of luck, he was third in this race last year on unsuitably soft ground and things went badly wrong in the Dubai Gold Cup in March.

Smartly away under Declan McDonogh, his saddle soon slipped and his rider was unable to ride any kind of finish, doing well in the end to be as close as he was.

Oxx said: "He's been in good form. He got off to a great start in Dubai – he was in great shape there but then the saddle slipped when he was in the form of his life. It might not have been an ideal prep for him and he has to go there without a recent run, but hopefully it will work out and he'll run well. He's been in better shape all round. Hopefully we'll have a good summer with him."

Oxx is pleased he decided not to run Saddler's Rock in the Saval Beg Stakes at Leopardstown earlier this month. "The Saval Beg used to be a better trial for the Gold Cup, but it's just 13 days beforehand now and is a bit too close," he said. "It's fine if you don't get too hard a race as a prep run, but it would have been a tough enough race close to the Gold Cup so we're happy we missed it."

Francois Doumen remains better known in Britain for his handling of top-class jumpers such as The Fellow and Baracouda, but in recent years has had plenty of success with his Flat runners. He saddles Group Two winner Top Trip, who was just beaten by a nose in the Yorkshire Cup by Glen's Diamond, with his trainer expecting enough improvement from York to make him competitive.

Doumen, who twice ran Kasbah Bliss in the Gold Cup, said: "I think the [soft] ground was the reason he ran so cool during the race at York. It shows how good he is that he was still nearly able to win. He has improved a lot since York, his rating has gone up a long way anyway. I think he will stay, he's so relaxed he should stay forever."

Willie Mullins' Simenon went into the history books 12 months ago when winning twice during Royal week, in the Ascot Stakes and Queen Alexandra Stakes. He was last seen finishing well to come fourth in the Chester Cup, having been saddled with an unfavourable draw.

His rider, Johnny Murtagh, said: "I was very impressed with him at Chester. We had a bad draw and we got a bit blocked on the inside, but I was very impressed with the way he finished.

"He likes Ascot, I know the Gold Cup is a step up from what he has been doing – he needs to improve seven or eight pounds – but he's the kind of horse that could do it. If he gets a nice run, he stays well. It's competitive, but he'll have to improve."

One-time Epsom Derby contender Vadamar represents Mikel Delzangles, having made a successful return this season with victory in a conditions race in May.

Delzangles said: "He's in good form and was very good on his comeback when winning at Longchamp. It's a different thing now in this race over a longer trip. He has had a long break so we will have to see if is ready to run a good race in the Gold Cup."

Last Train may be going into the race as an outsider, but connections believe this French challenger is also no forlorn hope. Teddy Grimthorpe, racing manager to owner Khalid Abdullah, is adamant the soft going was to blame for a poor performance from Andre Fabre's charge on his latest start at Longchamp.

On better ground, Grimthorpe reckons it could be a different story. "He was well beaten last time but had beaten those horses on better ground the time before and it was absolutely bottomless at Longchamp," he explained. "We know that he goes much better on fast ground. Of course two and a half miles finds everybody out, there's no hiding place there, but I think he's got a bit of class.

"He was second in the Juddmonte Grand Prix de Paris, which was a super run, and he always looked like a stayer. He has plenty of stamina in his pedigree and I think it's an open race this year."