After nearly a year of global travel and dozens of matches, most of Novak Djokovic's rivals are complaining about their mental and physical fatigue. Not the Serb, who thrives in the rigors of a tough autumn finish.

Unbeaten since his loss in the U.S. Open final to Rafael Nadal, Djokovic joined Nadal in the semifinals of the ATP World Tour Finals after overcoming strong resistance from Juan Martin del Potro on Thursday.

Tottenham, Fiorentina and Red Bull Salzburg made it four group wins out of four in the Europa League on Thursday to qualify for the knockout phase with two games to spare.

Record-signing Erik Lamela scored his first Tottenham goal before winning the penalty that Jermain Defoe converted to clinch a 2-1 victory over Sheriff Tiraspol in Group K.

Twitter's stock took to its wings in its public debut, closing up more than 70 percent. The day flew by with nary a hitch and gave birth to a new batch of Silicon Valley millionaires — even a few billionaires.

Now comes the hard part. With Twitter's value skyrocketing in just a day, the 7-year-old company that's never turned a profit and has just a fraction of Facebook's user base must prove to investors that it's worth the money. Twitter stock ended Thursday's trading at $44.90, giving the company a market value of $31 billion. That's $13 billion more than on Wednesday night, when the company set its IPO price at $26.

Kanye West has pleaded not guilty to misdemeanor battery and attempted grand theft in a case filed over a scuffle with a celebrity photographer earlier this year.

Attorney Blair Berk entered the plea on the rapper's behalf Thursday in a Los Angeles court. West was charged with two misdemeanors in September over a July altercation with paparazzo Daniel Ramos at Los Angeles International Airport.

Salvador Dali and Andy Warhol: one was a surrealist from Spain, the other, a pop artist from Pittsburgh.

They were among the most famous artists of the 20th century, and in 2014, their works will be on exhibit in adjoining galleries at a museum in Florida.

Chris Brown has countersued a man who accused the R&B star's entourage of attacking him outside a recording studio earlier this year.

The singer filed an assault and battery lawsuit Wednesday against Sha'keir Duarte, who claimed in an earlier suit that he was injured when a fight erupted between Brown and Frank Ocean's entourages in January outside a West Hollywood studio.

A Canadian bit-part actress barraged Alec Baldwin with lovesick, demanding messages, packed her belongings into a car and showed up uninvited at his homes in Manhattan and the Hamptons, prosecutors said as her stalking trial opened Thursday.

The case is expected to put the "30 Rock" actor in an unsought role: a key witness against a woman who says she had a fling with him some years ago, which he denies. Baldwin is expected to testify next week against actress Genevieve Sabourin, whose lawyer says she was merely trying to get answers after the actor lost interest in her.

Billionaire financier George Soros is pledging $1 million for humanitarian efforts in war-ravaged Syria.

Soros announced the pledge to the International Rescue Committee on Wednesday night after receiving its Freedom Award in recognition of his lifelong support for human rights and an open society.

The European Union moved closer to approving the cultivation of a second genetically modified corn on the continent despite years of objections by environmental groups and widespread apprehension about GMO food among European consumers.

Wednesday's approval by the EU Commission, the bloc's executive arm, now sends the plan to approve DuPont-Pioneer Maize 1507 to the EU's 28 member nations for consideration — and could lead to a decision on the issue within months.

A rocket carrying the Olympic flame successfully blasted off Thursday from earth ahead of the Sochi 2014 Winter Games.

NASA Live TV showed the rocket, emblazoned with the pale blue Sochi 2014 logo, launching from the Russian-operated Baikonur cosmodrome on a clear morning in Kazakhstan.