With this victory, Quins – boasting a number of Springboks in their ranks – passed the Vodacom Bulls on the log as the men from Pretoria are reliant on some results going their way in order to qualify for the playoffs.
Both teams were clearly intent on scoring tries, with six five-pointers in the first half, followed by a further three after the break.
With the emphasis seemingly on attack, the KwaZulu-Natalians were probably not as sharp on defence as they would have liked, while they also conceded too many penalties. As a result, Quins scored three tries in 12 minutes in the first quarter.
The game wasn't even 120 seconds old when Lukhanyo Am got the scoreboard ticking over with a brilliant try, from a little chip-and-gather into space, but then Danny Care (sixth minute), Alex Dombrandt (ninth minute) and Cadan Murley (18th minute) had the home supporters on their feet with great tries.
Leading by 21-8, Quins were cruising, but the Cell C Sharks weren't done yet and Jaden Hendrikse scored after a strong run from Marnus Potgieter before Eben Etzebeth showed his rugby intellect right before the break.
The big Bok lock saw Care had his hands on the ball at the back of a ruck just as Joe Marler disconnected ruck and he shot up, after which the Quins scrumhalf left the ball, allowing Etzebeth to snap it up and run 40m for a try that brought the Cell C Sharks back to a two-point deficit at the break.
But the hosts started the second half with a spring in their step as tries by Stephan Lewies, from an attacking lineout, and Nick David, from a brilliant grubber into space, saw them stretch their lead to 36-22.
The Cell C Sharks kept on pressing, but the harder they tried, the more mistakes crept into their game, and although Aphelele Fassi scored their fourth try two minutes from full time, they just could not catch Quins on the scoreboard.
Harlequins 39 (24) – Tries: Danny Care, Alex Dombrandt, Cadan Murley, Stephan Lewies, Nick David. Conversions: Marcus Smith (4). Penalty goals: Smith (2).
Cell C Sharks 29 (22) – Tries: Lukhanyo Am, Jaden Hendrikse, Eben Etzebeth, Aphelele Fassi. Conversions: Curwin Bosch (2), Lionel Cronje. Penalty goal: Bosch.