Kauto Star won a record fourth straight King George in 2009
Kauto Star made a winning start to the season with a four-length success in Down Royal's JNWine Champion Chase.
Ruby Walsh steered the 4-7 favourite, a dual Cheltenham Gold Cup winner, home ahead of fancied Sizing Europe and China Rock, both 5-1.
Two races later, however, Walsh broke his leg in a fall on Corrick Bridge.
The accident looks set to scupper the Irishman's bid to land a fifth consecutive King George VI Chase title on Kauto Star on Boxing Day at Kempton.
Walsh will have
surgery on fractures to the tibia and fibula in his right leg at Belfast's Royal Victoria hospital on Sunday.
"He jumped great, which is just what we wanted him to do and we can go on from here," said trainer Paul Nicholls.
It was Kauto Star's first run since a fall in the Gold Cup in March, which Nicholls said the 10-year-old was lucky to survive.
And Walsh was content to track the former Howard Johnson-trained Killyglen in the early stages as he helped force the pace with China Rock never far behind.
Mossbank, off the track for 927 days, loomed up large half a mile out but not surprisingly his absence eventually took its toll.
With the race hotting up, last season's Arkle winner Sizing Europe looked a huge threat to Kauto Star but try as Andrew Lynch might, he could never quite get on terms with the four-times King George winner.
Walsh's mount looked to be idling a little after the last but he was kept up to his work to beat Sizing Europe, who held off China Rock for second by a neck.
"They all thought it rode a good race and the ground was quite testing so we are chuffed to bits," said Nicholls.
"You never know after having a fall like he had whether they will lose their confidence, but we did a lot of work in the spring and Ruby rode him earlier in the week, his confidence is back. Stamina won the day.
"I shall know fairly quickly during the week whether he'll go for the Hennessy as I'll be able to tell how hard a race he had and what effect the journey has had.
"If he's back fresh and well and cantering he might go to the Hennessy, but if I find he's a bit quiet and I need to give him a couple of weeks off he won't go for the Hennessy."
JNWine Champion Chase (Down Royal)
1. Kauto Star (R Walsh) 4-7 Fav
2. Sizing Europe 5-1
3. China Rock 5-1
4. Coolcashin 20-1