Harlequins inflicted a first defeat of the season on Saracens in a stormy title showdown at Twickenham Stoop that also left the Aviva Premiership champions with concerns over George Kruis.
Kruis was struck with a swinging arm by James Horwill just 90 seconds into the match and play was delayed by eight minutes as the England lock was tended by medics and then carried off on a stretcher.
Horwill shook his head in disbelief when shown a yellow card by referee Craig Maxwell-Keys, but for the remainder of a feisty London derby won 29-23 by Quins the former Australia captain was magnificent and his reward came with an 80th-minute try.
Kruis' afternoon was over however and England, who had forwards coach Steve Borthwick present in the stands, will anxiously await an update on their concussed lock.
The 25-year was mentored by Borthwick at Saracens and was on course to forge an engine room partnership with Joe Launchbury for the RBS 6 Nations on the strength of his form this season.
Intriguing England duels abounded at Twickenham Stoop and a host of players are sure to have pleased Borthwick ahead of Eddie Jones' first squad announcement as head coach on Wednesday, among them Chris Robshaw, Jack Clifford, Owen Farrell, Danny Care and Marland Yarde.
Saracens' task in a captivating match was made harder when Rhys Gill was sent off in the 65th minute for his role in a spear tackle on centre George Lowe.
Gill was assisted by prop Petrus du Plessis as Lowe was picked up and dropped on his head, but it was the replacement hooker who was shown a red yard by Maxwell-Keys, who initially brandished a yellow before changing his mind upon repeat viewing of the incident.
It was a bruising encounter that saw Kruis, Duncan Taylor, Jack Clifford depart for head injury assessments but there were moments of brilliance, mainly by Quins - who scored dazzling tries through Rob Buchanan and Jack Clifford.
Saracens, Europe's only unbeaten team in Europe this season, arrived in south-west London in pursuit of a 16th successive victory and while they remain top of the table, their lead has been cut to three points.
Once the stricken Kruis had been carried from the pitch and play resumed, Saracens took advantage of Horwill's absence by kicking downfield and using their pack to squeeze out seven penalties while pounding the whitewash.
Scrum-half Neil de Kock completed the formality of touching down, but Quins' response was a brilliant line-out move that saw Charlie Matthews catch the ball and flick it to Care - who raced through the middle of a gap that had opened up.
Goode was moving across as the last line of defence so Care found Buchanan along the left touchline and the hooker scored.
Once again Saracens turned to their scrum, with number eight Billy Vunipola picking up and powering through tackles from Care, Evans and Luke Wallace to cross.
Quins discovered a new gear as half-time drew close, initially using their forwards to make ground with Robshaw and an inspired Horwill grinding out hard yards until Yarde produced the key breakthrough.
Yarde burst off his wing and forced a path to the whitewash before Evans gave Clifford the scoring pass while in mid-air, completing a thrilling try. Evans converted and Quins led 15-14 entering the interval.
There was no let up in the tension when the second half began as Farrell lifted England team-mate Mike Brown off the ground and dumped him to his back to roars of outrage from home fans.
Maxwell-Keys examined the incident on the big screen but deemed it legitimate.
Successive penalties from Farrell nudged Saracens back into a narrow lead, while in open play full-back Goode was becoming increasingly influential with his gliding runs into space.
Jones got the better of Mako Vunipola at a scrum, Ben Botica and Farrell exchanged penalties and Gill was sent off for his role in the spear tackle on Lowe.
A pulsating final 15 minutes unfolded with Botica's boot proving the difference, Robshaw and Clifford combining to foil an assault by Saracens, before Horwill burrowed over in the dying seconds.