Teenager Mascarenhas took six for 88 against Glamorgan at Southampton, the best figures by a Hampshire player on his debut this century, as well as having three catches dropped of his bowling.
He could not prevent the Welsh side reaching 401 all out, but Hampshire made an encouraging reply and were 193 for two at the close.
At the top of the table, Leicestershire had the most productive day among the title challengers as they headed towards a sizeable first innings lead against Nottinghamshire.
Vince Wells and skipper James Whitaker both scored centuries to steer their side to 341 for five, a lead of 17 with five wickets in hand after two days.
Second-placed Derbyshire piled up 524, their highest score against Somerset, but were then held up by opener Mark Lathwell who made a flawless 109 off 149 balls including 20 fours. At the close Somerset were 238 for four, still needing 137 to avoid the follow-on.
Essex were also made to struggle harder than expected after reducing Warwickshire to 14 for four at Edgbaston.
Left-hander Wasim Khan contributed his third century of the season as the defending champions recovered to 253 all out, claiming a first innings lead of 15, but Essex were back in charge on 113 for one at the close.
Lancashire, needing just 130 for victory over Middlesex on a sub-standard pitch at Old Trafford, were rocked by former England spinner Phil Tufnell in the closing overs.
Tufnell dismissed Jason Gallian and John Crawley for ducks, taking his tally of wickets for the match to eight and leaving Lancashire to resume in the morning on two for three.