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The pair will lead the Otago charge when they line up in the $50,000 super grand prix as part of the equestrian programme at Ride The Rhythm at Forsyth Barr Stadium on Friday night.
With the late scratchings of visiting North Islander Maurice Beatson's in-form My Gollywog and Ashburton rider Susie Hayward's stallion Andretti, the field was reduced to just five.
Both Hay, on Euro Sport Heartbreaker, and Whelan, on George Jetson, notched up perfectdouble clears to force a jump-off against the clock.
First out, Hay came home clear, putting the pressure on Whelan. Whelan went for a fast time, but a stop early on the course ended her chances.
Cave's Kelly Evans (Hammertime) finished third on four faults, ahead of Beatson on Conyers. His second ride, Schimmel Warrior, was eliminated in the first round.
For Hay, the win could not have come at a better time, filling her with confidence the stallion is on target for a strong showing against 32 of Australasia's top show jumpers on Friday.
''I was thrilled with him. I feel pretty confident. He feels fantastic,'' Hay said.
Honours in the pony grand prix went to Christchurch teen Steffi Whittaker, on Moonlight Glow, giving the pair a sixth individual grand prix win of the season.
Whittaker produced one of three double clears, forcing it to a jump-off.
Out first, Rakaia's Kimberley Bird went clear in a time of 63.41sec. Takapau's Emily Fraser, who won the junior rider event earlier in the day aboard Kiwi Relic, was next, taking a rail but finishing faster in a time of 51.98sec.
Whittaker, the last to go, finished in a nippy 51.42sec for victory.
Invercargill's Nicole White and Cricklewood Ace continued their successful run, winning the South Island young rider crown with the only clear round, as well as the 1.30m championship. Beatson was dominant in the age-group classes, winning the 7yr-old series with Parker and finishing first equal in the 5yr-old series with Mandalay Cove. The 6yr-old title went to Amanda Brown (Rangiora), on Glenmark.
This focus now switches to Dunedin for Ride The Rhythm, followed by two days of competition at the Taieri Agricultural Park in Mosgiel.
Equestrian Sports New Zealand yesterday named its national event squads, with one eye firmly on the 2016 Rio Olympics.
ESNZ high performance director Sarah Harris said the focus was on combinations which had the potential to produce an individual podium result in Rio.
Leading the way is world No 1 Andrew Nicholson, who has six horses on the squad. No 4 Jonathan Paget and No 6 Sir Mark Todd each have three.
Clarke Johnstone, formerly of Outram, London Olympians Caroline Powell and Jonelle Richards, and Olympic reserve Lucy Jackson round out the squad.
The international eventing season begins with the three-day Kentucky event in April.