But, either way, Melon slipped through on the inside and he turned for home a neck up on Buveur d'Air, an advantage he maintained until three quarters of the way up the hill.
It was a proper race and Melon is a good horse, but Buveur D'Air just put his head down and battled.
The Gordon Elliott-trained Mick Jazz was third at a price of 25-to1.
The winner's stablemate Charlie Parcs set a searing pace in the testing conditions and was followed by 2015 victor Faugheen before they both faded.
And jockey Lizzie Kelly scored her first Cheltenham Festival victory as 5-1 favourite Coo Star Sivola, trained by her stepfather Nick Williams, landed the Ultima Handicap Chase.
The big race of the day is the Champion Hurdle at 3.30pm and you'll struggle to find anyone going against Buveur D'Air, in the hunt for his 10th straight win.
Benie Des Dieux was second-best in the market at 9-2, with last year's victor Apple's Jade, a former Mullins inmate but now with Gordon Elliott, the 1-2 favourite. Owner JP McManus was also landing a seventh success in the race. "You saw that there, he's sensational".More news: Sealing drive: 1675 business establishments in Delhi sealed till Mar 9
In the Arkle Chase, Footpad made a serious early error but Walsh bided his time as Petit Mouchoir and Saint Calvados battled it out at the front and came through for an impressive 14-length victory on the 5-6 favourite. A jubilant Walsh, who returned to riding only a week ago after breaking his leg, punched the air and urged on the packed-out Cheltenham crowd after the win. "It's been a long winter and you wonder whether it's all worth it and then you win again and remember what it feels like", he said. "Physical pain can be managed".
The 6-year old finished just a neck ahead of 5-to-1 chance, Kalashnikov, with Mengli Khan in third under Jack Kennedy.
Victory for Kelly, the first woman to ride a Grade One victor over jumps - on Tea For Two in 2015 - was a form of Festival redemption.
"She really could be anything, I think a lot of her". "It was a long year last year, but I've had great years here so I can't complain".
The meeting began on the wettest ground for 36 years after persistent rain in the build-up, with the going described as heavy, soft in places.
A return to the winners' enclosure has been a long time coming for trainer Tom George, who was full of praise for only his second Festival victor.
He and Mullins completed a big race double when Benie Des Dieux prevailed in a three-way finish to the OLBG Mares' Hurdle at Cheltenham from Midnight Tour and Apple's Jade, the first odds-on favourite of the week to be beaten.