Ipswich warmed up for this week’s FA Cup replay against Saints by losing for only the second time in 22 games.
Chris Martin's 18th of the season - and 100th career goal - gave fellow Championship promotion hopefuls Derby victory at Portman Road after the striker latched onto a poor back header by Tommy Smith.
The Rams had to endure a nervy ending after midfielder John Eustace was sent off late on.
An even contest saw Ipswich players call for an early second-half penalty after Jay Tabb's cross appeared to hit the arm of Derby defender Richard Keogh.
But the breakthrough came on 57 minutes through Martin after he took advantage of a poor back header by Smith to nip in and score past ex-Saints keeper Bart Bialkowski.
The Pole has been a regular in recent months in the league for Ipswich, but was rested for last weekend’s FA Cup tie at his former employers.
After seeing his side lose for the first time in the league since October 21, Town boss Mick McCarthy questioned the performance of referee Kevin Wright.
When asked about the penalty appeal and also whether Eustace should have been sent off earlier after appearing to foul ex-Saint David McGoldrick, McCarthy said: “The ball has hit his arm and I think it was a penalty.
“As for Eustace, I asked the referee at half-time and he said he thought it was an accidental collision.
“But McGoldrick was barged from the side. I found that very strange - it could have changed the game.”
Summing up the match, McCarthy added: "Derby are a good side but it was a nothing game and certainly no spectacle.
“It probably should have ended 0-0 but we made a mistake (through Smith) and it cost us.
“Tommy has come in at the end and apologised but no apology was needed, we all make mistakes.”
Ipswich host Saints this Wednesday for the right to face Alan Pardew’s Crystal Palace in the fourth round.
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