United came from 2-0 down to lead 3-2 at half time, but then fell 4-3 behind before rescuing a point from a see-saw Ryman Football League Division One South encounter.
On a sunny but chilly afternoon, United got off to a bad start by falling behind in the first two minutes. Stephen Butcher's back header fell short of Lenny Pidgeley and Tashi Kwayie rounded the goalkeeper before tapping home.
Harry Stannard and Frannie Collin saw efforts saved by Grinstead goalkeeper Chris Haigh before United fell further behind in the 19th minute. Kwayie cut inside from the right and placed a lovely left-foot shot into the far bottom corner from outside the box.
United opened their account three minutes later in slightly fortunate fashion. Stannard got his boot to a diagonal ball from Kiernan Hughes-Mason and the ball hit the foot of the outcoming Haigh before cannoning back against Stannard and ricocheting into the net.
That was Stannard's last act before being substituted with a leg injury picked up early on. Jerson Dos Santos shot wide of United's goal from an inviting position inside the box before United equalised in the 38th minute.
Bradley Pritchard won possession in midfield and played a good ball through for Collin to round Haigh before finishing from an acute angle.
United then had to further shuffle the pack when Butcher was forced off and they needed a crucial block by Ollie Rowe from a goalbound Dos Santos shot to keep them on terms.
Hughes-Mason blazed over at the other end before putting United ahead in the seventh minute of added time. After receiving the ball from Collin, Hughes-Mason beat two defenders in the box before squeezing the ball home even though Haigh got a touch to the shot.
The momentum appeared to be firmly with United at the break, but Grinstead were the better side during the second half as their pace and movement in attack caused all sorts of problems for United's defence, particularly out wide.
Rowe made another important block and Mitchell Dickenson produced a timely intervention, while Charlie Harris wasn't far away with a shot from distance before Grinstead made it 3-3 on the hour. Dickenson couldn't prevent Kwayie lifting the ball over Pidgeley as he slid out and Marcus Elliott had a simple tap in.
Five minutes later Grinstead were in front once again. The sliding Pritchard tripped substitute Laste Dombaye in the box and Elliott tucked away the resulting penalty into the bottom corner.
Kwayie, who terrorised the United defence, drove just wide from 20 yards. Pidgeley then parried an angled Dos Santos shot and United broke down the other end to make it 4-4. Matt Bodkin played a good throughball for Hughes-Mason to slot home his second, despite Haigh getting a touch to his shot.
Somewhat surprisingly, there were no goals in the final 20 minutes, although United's goal lived a charmed life at times. Harris flashed a long range effort whiskers wide and Kwayie hit the base of the post with a curling shot from just outside the box.
Pidgeley pulled off a fantastic save low to his right to push aside Dombaye's deflected shot and a couple of minutes later made another great stop, this time with his legs to deny Elliott a hat-trick.
The result sees United drop a place to 10th in the table and they are now seven points outside the play-off spots, albeit having played fewer matches than all of the teams above them.
United: Pidgeley, Butcher (Dickenson 39), Thorbourne, Cruttwell, Rowe, Pritchard, Bodkin, Johnson, Hughes-Mason, Collin, Stannard (Harris 23). Subs not used: Dullaway, Nicholls. Attendance: 187.
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