Alberto Cavasin suffered his first home league defeat in charge of Leyton Orient, as they lost 1-0 to Portsmouth at the Matchroom Stadium.
Christian Burgess grabbed the only goal of the match, with a second half header to take all three points as the visitors move up to fourth in the table.
Cavasin made five changes to his side that were beaten 1-0 in the Checkatrade Trophy at Southend in midweek with Alex Cisak, Yvan Erichot, Jens Janse, Paul McCallum and Jay Simpson all stepped in.
Orient put Portsmouth under some early pressure, as Simpson went passed his marker and McCallum took the ball off his strike partner and his shot was stopped by the feet of David Forde.
The visitors then had a spell themselves, as Conor Chaplin slotted the ball into the bottom corner but the linesman flagged offside.
Orient were forced into an early change, as Captain Kelly had to be stretchered off due to an injury just five minutes in.
The home fans called for a penalty, after a challenge on Erichot inside the six-yard box, which was not given by the referee.
Alex Cisak almost gifted Portsmouth the lead, as he passed the ball straight to Gary Roberts who squared the ball to Chaplin but strike was saved by Cisak – who make up for his mistake.
Portsmouth’s Kyle Bennett went down inside the box, which the referee looked to have pointed to the spot but gave a free-kick to Orient and booked the striker for simulation.
In the second half, Orient made their second change as Alan Dunne replaced McCallum and the home side shifted their formation from 4-4-2 to 4-5-1.
Newly introduced Dunne nearly make the perfect introduction, as he intercepted the Portsmouth midfielder and went to lob one of his former teammates Forde but he was equal to it.
Portsmouth then took the lead just before the hour mark, as Carl Baker’s corner was headed in by Stevens into the bottom corner.
The away side were nearly gifted a second, as a harmless cross was nearly headed in by Nicky Hunt as it came back off the woodwork at the near post.
Orient pushed for an equaliser but Portsmouth held off the late pressure to stop their run of back-to-back defeats, with Orient now lying just above the relegation zone.
Leyton Orient Lineup: 1 Alex Cisak, 16 Nicky Hunt, 37 Yvan Erichot, 6 Tom Parkes, 3 Callum Kennedy, 46 Jens Janse, 11 Gavin Massey, 4 Liam Kelly © (9), 18 Ulrich Nnomo (82), 10 Paul McCallum (45), 27 Jay Simpson
Substitutes: 24 Sam Sargeant, 25 Aron Pollock, 5 Alan Dunne (45), 33 Zan Benedicic, 22 Sandro Semedo (9), 21 Jordan Bowery, 9 Ollie Palmer (82)
Portsmouth Lineup: 1 David Forde, 3 Enda Stevens, 5 Matt Clarke, 6 Christian Burgess, 7 Carl Baker, 8 Michael Doyle ©, 11 Gary Roberts, 19 Conor Chaplin (67), 23 Kyle Bennett, 24 Amine Linganzi (67), 26 Gareth Evans
Substitutes: 13 Liam O’Brien, 2 Tom Davies, 4 Danny Rose (67), 9 Michael Smith, 10 Milan Lalkovic (82), 14 Curtis Main (67), 22 Kal Naismith