Azertyuiop put in a superb performance to take the Queen Mother Champion Chase after the favourite Moscow Flyer fell with four fences to go.
It was a sweet victory for jockey Ruby Walsh and trainer Paul Nicholls after finishing second to Moscow Flyer at Sandown in December.
Earlier, Rule Supreme upset the odds to claim victory in the Royal & SunAlliance Novices Chase.
The horse, ridden by David Casey, came in at 25-1, beating the well-fancied Our Vic and Tony McCoy, who eventually finished third.
1745 Weatherbys Champion Bumper (2m ½f)
1. Total Enjoyment (J Culloty) 7-1
2. Refinement (B Geraghty) 7-1
3. Master Albert (JP Magnier) 50-1
24 ran. Dist: 2½, 5
Jim Culloty warmed-up for Thursday's Gold Cup with victory in the Bumper on the impressive-looking Total Enjoyment.
Once Culloty took over the lead coming up the hill, he easily managed to hold off the challenge of Barry Geraghty on Refinement for another Irish winner.
JP Magnier, the son of John Magnier, who finished third in the Coral Cup on Rhinestone Cowboy, finished well for third on Master Albert.
The victorious Culloty gets a huge reception when he returns to the winners' enclosure.
1710 Mildmay of Flete Challenge Cup Handicap Chase (2m 4f)
1. Tikram (TJ Murphy) 12-1
2. Iznogoud (AP McCoy) 9-2
3. Fondmort (MA Fitzgerald) 9-1
4. Kelrev (S Thomas) 16-1
17 ran. Dist: 2½, 1½
Tikram and Timmy Murphy used their speed on the run-in to outpace their rivals for Murphy's second win of the Festival.
Tikram was behind for most of the race and it looked like he would finish well down the field.
Coming to the last, Tony McCoy and Iznogoud looked strongest but Tikram put in a superb burst of speed coming up the hill to win with Fondmort and Mick Fitzgerald defying top weight for third.
1635 National Hunt Steeplechase Challenge Cup (4m)
1. Native Emperor (Mr R Widger) 5-1jf
2. Celestial Gold (Mr A Farrant) 33-1
3. Drombeag (Mr JT McNamara) 8-1
21 ran. Dist: 1¾, 6
Robert Widger swooped late on the Jonjo O'Neill-trained joint favourite, overcoming Ashley Farrant on Celestial Gold on the run-in for a well-earned win.
Celestial Gold broke away of the rest of the field a couple of fences from home as Native Emperor was held up by his jockey but when Widger let him go, he went strongly through the field.
Celestial Gold found the Cheltenham hill too much and had to be content with second while another Jonjo O'Neill-trained horse, Drombeag, finished third.
1600 Coral Cup Handicap Hurdle (2m 5f)
1. Monkerhostin (R Johnson) 7-1
2. Court Shareef (W Hutchinson) 200-1
3. Rhinestone Cowboy (JP Magnier) 5-1
4. Mughas (R Thornton) 11-2
27 ran. Dis: 1, 1¼
Monkerhostin gave Richard Johnson his first winner at the 2004 Festival with an excellent display in the Coral Cup.
Plenty of horses were in with a chance coming to the last in what was a keenly-contested race.
But the Philip Hobbs-trained horse held on up the hill to stay ahead of 200-1 chance Court Shareef with last year's Champion Hurdle challenger Rhinestone Cowboy back in third.
1520 Queen Mother Champion Chase (2m)
1. Azertyuiop (R Walsh) 15-8
2. Flagship Uberalles (R Johnson) 14-1
3. Tiuchev (AP McCoy) 16-1
8 ran. Dist: 9, 1½
Last year's Arkle winner Azertyuiop, ridden by Barry Geraghty claimed a very impressive win in the Queen Mother Champion Chase.
There was disappointment for the Irish when 5-6 favourite Moscow Flyer unseated jockey Barry Geraghty four fences from home when well in contention.
The Paul Nicholls-trained mount took control of the race from then on and jumped superbly, extending his advantage to claim a famous nine-length victory.
Richard Johnson is a brave second on Flagship Uberalles with Tony McCoy third on Tiuchev.
1435 Royal & SunAlliance Novices' Chase (3m ½f)
1. Rule Supreme (D Casey) 25-1
2. Royal Emperor (D Elsworth) 6-1
3. Our Vic (AP McCoy) 11-8f
10 ran. Dist: 1¼, 2
The Willie Mullins-trained Rule Supreme, ridden by David Casey became the latest big-priced winner with a surprise but welcome victory for the Irish contingent on St Patrick's Day.
The favourite Our Vic jumped disappointingly but still led for much of the race before being overhauled coming to the final fence.
Up to then, Our Vic and the winner's stablemate Mossy Green (who fell at the second last) had made the running but coming to the last, Rule Supreme and Royal Emperor started to breathe down his neck and mount their challenge.
1400 Royal & SunAlliance Novices' Hurdle (2m 5f)
1. Fundamentalist (C Llewellyn) 12-1
2. Inglis Drever (G Lee) 7-4f
3. Grey Report (R Johnson) 25-1
15 ran. Dist: ½, 4
Fundamentalist dug in deep to hold off the challenge of the favourite Inglis Drever up the Cheltenham hill to win the opening race on the second day.
Carl Llewellyn kept his mount well-placed throughout the race and took the lead from long-time leader Grey Report with two fences to go and built up a useful lead.
But Graham Lee brought the well-fancied Inglis Drever up to challenge but although he cut the gap as Fundamentalist tired, he couldn't get past his rival with Grey Report and Richard Johnson hanging on well for third.
It was the experienced Llewellyn's first Cheltenham winner for 10 years and opens trainer Nigel Twiston-Davies' 2004 Festival account.