West Ham United were held to a 2-2 draw by Romanian side AFC Astra Giurgiu despite Enner Valencia and Mauro Zarate firing them into a two-goal lead.
The home side looked to be in pole position after Valencia opened the scoring with a close-range header and Zarate scored a superb individual goal early in the second half.
But following the dismissal of James Collins for two bookable offences, Astra Giurgiu made the extra man count and a fine strike from Fernando Boldrin and a late own goal from Angelo Ogbonna means the tie is wide open heading into the second leg in Romania next week.
West Ham United also suffered a major injury blow when Valencia was forced to leave the field on a stretcher in the first half with a leg injury.
With Diafra Sakho and James Tomkins already serving a suspension in the competition, manager Slaven Bilic cannot afford to lose any more of his players as he prepares for the start of the new season.
After a quiet opening ten minutes to the game, the crowd soon got into life as Valencia burst clear and played the ball into Cheikhou Kouyate who made a powerful surge into the penalty box but the ball was just taken away from him at the vital moment as the Astra defence cleared for a corner.
The home side came close again on 17 minutes when Payet's corner picked out Zarate who saw his volley superbly blocked by the Astra defence. The ball rebounded to the Argentine forward who saw his deflected shot just wide of the post.
It was not all one-way traffic and Astra Giurgiu alsio came close to breaking the deadlock on 20 minutes when William De Amorim played the ball to Filipe Teixeira who found himself unmarked inside the box and drilled a powerful low shot which flew inches past the post.
But it was West Ham United who opened the scoring on 23 minutes and it was that man Payet who played a crucial role in setting up the first goal.
The France international weaved his way into the area before picking out the perfect ball for Valencia who made no mistake directing a header into the corner of the net.
Payet was now clearly in the mood and almost doubled West Ham's lead on 27 minutes when he picked up a loose ball on the edge of the box and fired a shot which flew just over the bar.
O'Brien was booked for a foul on Takayuki Seto and will now face a one match ban after picking up three cautions in the competition this season.
The West Ham defender was then forced to leave the action on 34 minutes when he picked up an injury and had to be replaced by Reece Burke.
There was more bad news for the home side sixty seconds later as Valencia was then forced to leave the field on a stretcher after picking up a leg injury.
The injury looked unpleasant and he was taken straight down the tunnel for further treatment.
The Ecuador international had started the game so well and seemed to be forging a great partnership with Payet, but he was replaced by Modibo Maiga.
West Ham continued to push for a second goal and James Collins was unlucky not to get on the scoresheet just before half-time when his glancing header flew just over the bar.
His centre half partner Ogbonna also came close to getting on the scoresheet as his close-range header produced a fine save from Astra keeper diving away to his left hand side.
West Ham had a strong finish to the first half and Zarate was unlucky not to get his name on the scoresheet when his curling shot from the edge of the box was superbly tipped away by Lung.
West Ham continued to create pressure on the Astra goal in the early stages of the second half and Kouyate fired just over.
But the home side were finally rewarded for all their dominance when Zarate made it two in spectacular fashion on 51 minutes.
The Argentine forward showed his quality once again when he skipped his way past four challenges and made no mistake striking the ball into the back of the net for his first competitive goal at the Boleyn Groiund.
But the good luck ended there as West Ham United were then reduced to ten-men on 58 minutes when Collins was sent off for a second bookable offence for a foul on Budescu.
The Wales international could not believe his luck when Swiss referee Adrien Jaccottet pulled out the red card.
Astra almost pulled back a goal immediately when Mark Noble cleared the ball off the line from Fernando Boldrin's corner on 63 minutes.
But the away side made their extra man count when Fernando Boldrin pulled the Romanian team back into the game with a stunning strike on 71 minutes.
Boldrin picked up the ball unchallenged and fired an unstoppable shot which sailed past Adrian into the roof of the net.
Things went from bad to worse for the Hammers on 81 minutes as Astra Giurgiu drew level after Ogbonna could only slice a cross from the left hand side past his own keeper and into the corner of the net.
Bilic was then given his marching orders in injury time and was asked to leave the dug-out by referee Jaccottet.
West Ham United will now have it all to do in Romania next Thursday if they are to reach the play-off round in the Europa League.
West Ham United: Adrian, Cresswell, Kouyate, Zarate (Jarvis 76mins), Noble (c), O'Brien (Burke 36mins), Collins, Ogbonna, Payet, Valencia (Maiga 37 mins), Oxford
Subs: Spiegel, Nolan, Poyet, Samuelsen
Astra: Lung, Alves, Seto, Enache, Budescu (Lovin 90mins), Morais, Boldrin (Dandea 87mins), Gaman, Teixeira, Quieros, De Amorim (Florea 76mins)
Subs: Gavrilas, Oros, Alves, Stan