Leicester 9 - 22 Harlequins
Fly-half Nick Evans helped Harlequins inflict a first home defeat on Leicester for almost a year.
The champions showed that they will not be giving up their title without a fight after emerging from a tight contest with a 22-9 victory, their second consecutive win at Welford Road following their 28-17 success last October.
The victory was Quins' fourth on the bounce with Evans kicking four penalties to keep the visitors ahead, after he also converted the only try of the match from Tom Williams on 20 minutes - a score which had given Harlequins the advantage following two early Toby Flood penalties.
Danny Care also chipped in with a drop goal to give Quins a 13-6 half-time lead and although Flood, reduced the arrears with his third penalty early in the second half, the boot of Evans responded with two of his own to ensure his side maintained their unbeaten start to the campaign.
Harlequins have moved a point clear of Northampton at the top of the embryonic Aviva Premiership table, while Leicester now find themselves down in third after being unable to score a try for the second match running.