Modric scored in the first half and then Defoe registered his fourth goal against his former club with eight minutes left to send Spurs back into the top four of the Premier League.
Portsmouth had no choice but to sell Peter Crouch to TottenhamPremier League team news: Weekend squad, stats and betting tipsIn the final game of the decade at White Hart Lane, Spurs made four changes, with Modric starting for the first time since August when he broke a bone in his leg against Birmingham.
Modric replaced Niko Kranjcar, while Ledley King was back for his first start since early November as a replacement for Sebastien Bassong, who suffered a head injury at Fulham on Saturday.
Harry Redknapp also recalled Defoe, for Robbie Keane, while Tom Huddlestone replaced Jermaine Jenas in midfield.
West Ham, meanwhile, were forced to make one change from the team that won three crucial points against fellow strugglers Portsmouth, with Mark Noble injured and Valon Behrami taking his place for his first start for five weeks.
However, West Ham received a blow early in the game when Scott Parker, outstanding in recent weeks, pulled up with a recurrence of a hamstring problem and had to be replaced.
Parker had forced the game's first chance, a wayward shot, but in so doing he suffered the problem. Yet West Ham were slow to replace him and with Luiz Jimenez waiting to come on, Tottenham took the lead.
Radoslav Kovac could have kicked the ball out, but loss possession, and in the 11th minute Defoe fed Aaron Lennon, whose cross from the right bounced off a defender and fell to Modric, who responded with a half volley that beat Robert Green.
Gianfranco Zola, the West Ham manager, was forced to make another change with only 19 minutes played, with Jonathan Spector replacing Herita Ilunga at left-back, who suffered a similar problem to his injured team-mate.
In the 26th minute West Ham rallied when Jack Collison forced Heurelho Gomes to save well. Either side of that effort came two attempts from Huddlestone, both going wide.
The first half pressure was maintained when Wilson Palacios' cross was touched narrowly wide by Peter Crouch. And before the interval Defoe came close with a 30 yard dipping free-kick that sailed just off target.
For the second half West Ham pushed Jimenez forward alongside Guillermo Franco, but Spurs were first to threaten with Huddlestone and Lennon both having pops at Green's goal early on.
In the 56th minute, Spurs could have scored again when Huddlestone's shot struck the foot of the left post and Modric sent the rebound high.
West Ham received a further blow when, midway through the second half, Franco was booked for fouling Palacios and, as a result, will be banned from Saturday's FA Cup tie against Arsenal, leaving them short on attacking options.
And Spurs, who had looked nervous, responded when Defoe struck a shot against the chest of Green and then responded by firing the rebound majestically into the top corner.