LONDON — Chelsea was frustrated by Southampton's dogged defending and Fraser Forster's saves on Sunday, going six points clear of Manchester City with a 1-1 draw but missing the chance to open up a bigger lead at the Premier League summit.
Against an energetic and creative Southampton side, Chelsea seemed to experience a Champions League hangover in the first half, four days after being eliminated by Paris Saint-Germain.
Although Chelsea striker Diego Costa ended a seven-game goal drought, Southampton quickly leveled from the spot through Dusan Tadic in the 19th minute and seized the initiative.
"We showed that when we play good as a team nobody can beat us," Tadic said.
Goalkeeper Thibaut Courtois came to Chelsea's rescue with instinctive saves in the first half. And Chelsea was then repeatedly thwarted after the break by Forster demonstrating why Southampton has conceded the fewest goals this season.
"The penalty changed the rest of the first half and my players felt it," Chelsea manager Jose Mourinho said. "We stopped playing and we made a few mistakes. They scared us with a few dangerous counterattacks.
"From the moment Ramires came on (for Nemanja Matic in the 53rd minute), we killed their counterattack. They felt that and we had complete control."
But after opening the league season with 10 successive home wins, Chelsea has drawn three of its last four at Stamford Bridge.
It would take an epic collapse for Chelsea to miss out on its first league title since 2010. The Blues have a six-point cushion at the top, and a game in hand over second-place defending champion City, which was surprisingly beaten by Burnley on Saturday.
When Chelsea's Champions League campaign ended, the inquest highlighted Costa's failure to score once in Europe this season for Chelsea.
So prolific in front of goal in the first half of the domestic season, Costa's finishing has evaded him. But the Spain striker just took 11 minutes to start and complete the move for his league-leading 18th goal of the season.
Eden Hazard, released by Costa, held off Jose Fonte before sending the ball out wide to Branislav Ivanovic, who provided a pinpoint cross for Costa's header.
The lead was immediately protected by Courtois pushing away Sadio Mane's shot. But Mane was tripped by Nemanja Matic shortly afterward in the penalty area. Tadic sent the ball down the middle, going into the net after clipping Courtois' feet.
Chelsea's own penalty appeal was dismissed by referee Mike Dean when Ivanovic went down after being clipped by Tadic.
Southampton's bursts forward were unsettling the hosts, with Mane particularly menacing and Tadic lively.
Courtois preserved parity before half time by denying Tadic and diving low to block Mane's strike. And at the start of the second half, the Belgium goalkeeper palmed away one-handed a free kick from Toby Alderweireld, and Mane's follow-up overhead kick was also blocked.
But then Mourinho believes Chelsea "recovered tactically and emotionally."
Costa was looking determined to make up for his recent barren spell in front of goal, appealing for a penalty after being fouled by Fonte and then trying his luck with an overhead kick from Hazard's cutback.
Southampton's rearguard was superb, with Chelsea finding no way through.
Forster pulled off a double save midway through the half — scooping away Oscar's header and batting clear a follow-up from Hazard — then repelled attempts from Cesar Azpilicueta and substitute Loic Remy in the final minutes.
"He played an unbelievably high level for the team," Southampton manager Ronald Koeman said of Forster.
The point keeps Southampton in touch with an unexpected challenge for the top four Champions League places.