(CNN) -- As the swimming meet comes to its final days, the action at track and field begins at the Olympic Stadium. Here's what to look for as that competition starts and in four other notable sports:
1) Track and field: Sprint queens begin their quests
A few weeks ago, Allyson Felix watched as Jeneba Tarmoh took a celebratory lap at the Olympic Trials, apparently the third qualifier for the Olympics in the women's 100 meters. But Friday, it's Felix who will take to the blocks in a quest to finally win a gold medal. There was a second look at the photo finish. The two sprinters had tied, officials ruled. Then the track bosses scrambled to figure out a way to break the tie and decide who would run the event in London. On the day of a scheduled runoff, Tarmoh backed out. She's still part of the U.S. team, but she's a member of the relay pool. On Friday, Felix will race twice, seeking a spot in one of Saturday's semifinals.
Track and field begins at 10 a.m. London time (5 a.m. ET)
2) Swimming: Speaking of photo finishes
Four years ago, it became one of the most famous photos of the Beijing Games: Shot from the bottom of the pool, Michael Phelps in one lane, Milorad Cavic in the other, two swimmers lunging for Olympic gold. Phelps won the 100-meter butterfly that day by .01 seconds, after catching the Serbian swimmer gliding to the wall as Phelps took a quick, final stroke. Tonight, they will race again. But also look out for Chad le Clos, who beat Phelps in the last few meters of the 200-meter butterfly.
Swimming begins at 10 a.m. London time (5 a.m. ET); the medley relay heats start at 11:49 a.m. in London (6:49 a.m. ET) followed by four finals in the 7:30 p.m. London time session (2:30 p.m. ET).
3) Tennis: Hopes for happier result at home
Britain's Andy Murray, who reached Wimbledon finals a few weeks ago, plays Novak Djokovic of Croatia in one men's singles semifinal. In the other, Roger Federer of Switzerland matches up with Juan Martin del Potro. The women also play singles semifinals; those matches are Serena Williams of the United States vs. Victoria Azarenka of Belarus, and Russians Maria Sharapova vs. Maria Kirilenko.
Both matches begin at noon in London (7 a.m. ET)
4) Soccer: A big Hope here, too
The United States is backed by goalie Hope Solo, who gave up two criticized goals early to France and nothing since. The Americans face New Zealand (nicknamed the Football Ferns), which finished third in Group E. In February the Football Ferns nearly beat the United States, but forward Alex Morgan scored two late goals to rally the Americans. They also played a closed door match a few days later; no results were reported. The highlight match of the day could be Brazil with its great player Marta vs. World Cup champion Japan.
Matches start at noon London time (7 a.m.) with the U.S.-New Zealand match at 2:30 p.m. London time (9:30 a.m. ET)
5) Judo: Saudi female expected to wear headscarf
At first, judo officials said they wouldn't allow 16-year-old Wojdan Shaherkani of Saudi Arabia to wear a hijab during her judo match. Shaherkani's father and Saudi officials insisted she would not compete unless she could wear the headscarf. On Friday, she will be permitted to wear it when she fights Melissa Mojica of Puerto Rico in a round of 32 match. Shaherkani, who is competing at her first major international event, received a special invitation to compete from the International Olympic Committee.
Shaherkani is scheduled to fight at 10:33 a.m. London time (5:33 a.m. ET).