Groombridge soon caught up, however, passing Greenslade for the lead when the Auckland man crashed on lap three and Groombridge did not look back from there as he steadily increased his advantage to the finish.
Greenslade (KTM) finished runner-up, the third time consecutively he's had to follow Groombridge across the finish line this season, and he looks safe to finish the championships in the runner-up spot.
Third overall in the senior standings after three rounds is Taupo's Nathan Tesselaar (KTM), while Raglan's Jason Dickey (KTM) and Te Kauwhata's Jacob Brown (KTM) round out the top five.
Meanwhile, the 90-minute junior cross-country race was held earlier in the day on Sunday.
That was won by Te Awamutu's Daniel White (Kawasaki), with Raglan's Logan Shaw (Honda) and Cambridge's Seton Head (KTM) completing the podium positions. Raglan's Coby Rooks (Honda) and Eltham's Adam Loveridge (Husqvarna) rounded out the top five.
With a 3-2-3 score-card from the three rounds, Head leads the title chase by eight points from Rooks (2-7-4), with White (1-dns-1) now up to third overall, although he is also in the strongest position now to take the title.
With riders discarding their worst result, White will toss out his no-show result from round two, meaning a finish anywhere inside the top five at the final round should earn him the title for 2017.
While Shaw is down in fifth overall after three rounds, his 23-1-2 score-card could also be enough to claim the title. He'll discard his 23rd place finish, after he crashed out while leading at round one, and set his sights on winning the final round to compile a title-clinching result.
The fourth and final round of the cross-country nationals is set for Cambridge on May 13.