Marcus Smith's superb display helped Harlequins beat Top 14 league leaders Racing 92 in a thrilling Investec Champions Cup pool match in Paris.

Nolann le Garrec and Ibrahim Diallo put the hosts ahead before Quins skipper Alex Dombrandt gave his side hope.

Hooker Jack Walker went over moments later to seal a superb bonus-point win.

The visitors celebrated with gusto after a hard-earned victory against in-form opponents boasting South Africa's double World Cup-winning captain Siya Kolisi and England back Henry Arundell in their ranks.

Smith was the star of the show, displaying all his usual incisive attacking abilities but also producing one try-saving tackle in a superb defensive effort.

Harlequins' win meant English sides have won seven of their eight matches in the opening round of the tournament, with only Saracens falling to defeat.

Tabai Matson's side will look to keep their Pool 2 momentum going when they face Toulouse at the Stoop next weekend, while Smith will bid to further enhance his claims to start at fly-half for England after playing at full-back for most of the World Cup.

Smith inspires Harlequins to first-half lead

Harlequins, who thrashed Premiership leaders Sale last weekend, thought they had an early try from scrum-half Danny Care at the spectacular domed Paris La Defense Arena.

Dino Lamb bounced off defenders in a rampaging run before Care dived over, but the try was disallowed for a double movement by the Italian second row.

Racing 92 responded well, with giant lock Cameron Wiki powering past Care to set up scrum-half Le Garrec for their first try but Quins were soon level.

Smith sold Racing 92's defence a huge dummy and sprinted past the covering players to touch down under the posts before converting.

A few minutes later, Andre Esterhuizen smashed through a defender and ran clear to extend the visiting side's lead.

Revelling in the occasion, Smith then fired over an audacious drop-goal from near halfway.

But an action-packed first half finished with the hosts' Antoine Gibert stretching over in the corner to cut the deficit to just three points.

Quins fight back to secure bonus-point win

Racing started at pace after the break and the livewire Le Garrec touched the ball down with his fingertips to put his side into the lead.

A brilliant last-ditch tackle from Smith prevented Juan Imhoff from scoring a spectacular try in the corner as the hosts ramped up the pressure.

Racing's sustained effort finally paid off when Diallo fought his way over to secure a bonus-point fourth try.

After Esterhuizen punched another hole in Racing's defence, Dombrandt burst through a gap and held off two tacklers to give Quins hope.

And seizing the moment once again, Smith chose to kick for the corner and replacement hooker Walker ended up rumbling over the line from a rolling maul.

Racing launched one last bid for victory but Harlequins, despite having Lamb sin-binned in the final minute, held off everything the French side threw at them to claim a famous victory.

Line-ups

Racing: Arundell; Habosi, Fickou, Saili, Imhoff; Gibert, Le Garrec; Gogichashvili, Tarrit, Nyakane, Palu, Woki, Diallo, Kolisi, Lauret.

Replacements: Arous, Nyakane, Gomes Sa, Chouzenoux, Joseph, Le Bail, Tedder, Chavancy.

Harlequins: Green; David, Joseph, Esterhuizen, Murley; Smith, Care; Marler, Riley, Lewis, Launchbury, Lamb, Cunningham-South, W Evans, Dombrandt (capt).

Replacements: Walker, Baxter, Kerrod, Herbst, Chisholm, Porter, J Evans, Beard.

Sin-bin: Lamb (79)

Referee: Frank Murphy