By Mike Collett

LONDON (Reuters) - Mexico will play Japan in the semi-finals of the men's Olympic soccer after beating Senegal 4-2 after extra time in a thrilling quarter-final that delighted a crowd of almost 82,000 at Wembley on Saturday.

Mexico, 2-0 up midway through the second half, were pegged back to 2-2 by the African debutants before Giovani Dos Santos put them back in front after 98 minutes when he pounced on an error by Papa Gueye to fire home for his third goal of the tournament.

Substitute Hector Herrera made it 4-2 after 109 minutes when he headed home following some more loose defending to put Mexico into the last four for the first time since they hosted the Olympics in 1968.

Japan also made it through to the last four for the first time since they beat Mexico in the bronze medal match in 1968 after keeping a fourth consecutive clean sheet with a 3-0 win over Egypt.

The north African side were reduced to 10 men in the 41st minute at Old Trafford in Manchester, where 70,000 fans turned up for the often maligned men's Olympic soccer tournament.

The other two semi-finalists will be known later on Saturday after gold medal favorites Brazil face Honduras in Newcastle (1600 GMT) and hosts Britain play South Korea in Cardiff (1830 GMT).

The Japan-Mexico semi-final will be at Wembley on Tuesday with the other game at Old Trafford on the same night.

MEXICO CRUISING

With just over an hour played Mexico appeared to be on their way to the last four when they led 2-0 after going ahead early when lanky, shaven-headed Jose Enriquez flicked home a free kick from Dos Santos after 10 minutes.

They doubled their lead after 63 minutes when Javier Aquino pounced on a loose ball in the area and rifled it home.

Senegal, so impressive in the group stage, were never out of contention though and after plenty of chances of their own pulled one back when Moussa Konate headed home a Pape Souare cross after 69 minutes.

It was Senegal's fifth goal of the tournament and he had scored all five.

But seven minutes later substitute Ibrahima Balde joined him on the scoresheet when he thundered in an unstoppable header from a Souare corner to pull Senegal level.

Both goalkeepers - Jose Corona of Mexico and Ousmane Mane of Senegal - made some outstanding saves to delight a hugely appreciative crowd of 81,855 as the game flowed from end to end throughout two superb hours of entertaining soccer.

JAPAN THROUGH

In the first match of the day Kensuke Nagai put Japan ahead after 14 minutes at Old Trafford and they wrapped the game up in the last 12 minutes with headers from Maya Yoshida and Yuki Otsu.

They took full advantage after defender Saad Saadeldin was sent off in the 41st minute for a professional foul on Manabu Saito.

Japan, whose world champion women's team have also reached the semis in their competition, went ahead when Nagai took advantage of a mix-up between Egypt goalkeeper Ahmed Elshenawi and his defense to pounce for his second goal of the tournament.

(Editing by Mark Meadows)